Remove ReadmeBlob since it isn't used

ReadmeBlob#rendered_markup isn't called, but intended to be
called by BlobViewer::Markup. ReadmeBlob uses SimpleDelegator
to delegate all missing methods to Blob, but when the simple_viewer
call bubbles up to Blob#simple_viewer, the class isn't ReadmeBlob
anymore, but Blob, so it doesn't respond to rendered_markup.

ReadmeBlob was only responsible for calling
Repository#rendered_markup that cached the content in Redis.
Currently, we cache every blob content in Redis, so we can
remove ReadmeBlob instead of fixing the above problem.

app/models/readme_blob.rb

-# frozen_string_literal: true
-
-class ReadmeBlob < SimpleDelegator
-  include BlobActiveModel
-
-  attr_reader :repository
-
-  def initialize(blob, repository)
-    @repository = repository
-
-    super(blob)
-  end
-
-  def rendered_markup
-    repository.rendered_readme
-  end
-end

app/models/repository.rb

@@ -39,7 +39,7 @@ class Repository
   #
   # For example, for entry `:commit_count` there's a method called `commit_count` which
   # stores its data in the `commit_count` cache key.
-  CACHED_METHODS = %i(size commit_count rendered_readme readme_path contribution_guide
+  CACHED_METHODS = %i(size commit_count readme_path contribution_guide
                       changelog license_blob license_key gitignore
                       gitlab_ci_yml branch_names tag_names branch_count
                       tag_count avatar exists? root_ref merged_branch_names
@@ -53,7 +53,7 @@ class Repository
   # changed. This Hash maps file types (as returned by Gitlab::FileDetector) to
   # the corresponding methods to call for refreshing caches.
   METHOD_CACHES_FOR_FILE_TYPES = {
-    readme: %i(rendered_readme readme_path),
+    readme: %i(readme_path),
     changelog: :changelog,
     license: %i(license_blob license_key license),
     contributing: :contribution_guide,
@@ -498,23 +498,7 @@ class Repository
   end
 
   def blob_at(sha, path)
-    blob = Blob.decorate(raw_repository.blob_at(sha, path), container)
-
-    # Don't attempt to return a special result if there is no blob at all
-    return unless blob
-
-    # Don't attempt to return a special result if this can't be a README
-    return blob unless Gitlab::FileDetector.type_of(blob.name) == :readme
-
-    # Don't attempt to return a special result unless we're looking at HEAD
-    return blob unless head_commit&.sha == sha
-
-    case path
-    when head_tree&.readme_path
-      ReadmeBlob.new(blob, self)
-    else
-      blob
-    end
+    Blob.decorate(raw_repository.blob_at(sha, path), container)
   rescue Gitlab::Git::Repository::NoRepository
     nil
   end
@@ -612,15 +596,6 @@ class Repository
   end
   cache_method :readme_path
 
-  def rendered_readme
-    return unless readme
-
-    context = { project: project }
-
-    MarkupHelper.markup_unsafe(readme.name, readme.data, context)
-  end
-  cache_method :rendered_readme
-
   def contribution_guide
     file_on_head(:contributing)
   end

spec/lib/gitlab/repository_cache_adapter_spec.rb

@@ -292,12 +292,11 @@ RSpec.describe Gitlab::RepositoryCacheAdapter do
 
   describe '#expire_method_caches' do
     it 'expires the caches of the given methods' do
-      expect(cache).to receive(:expire).with(:rendered_readme)
       expect(cache).to receive(:expire).with(:branch_names)
-      expect(redis_set_cache).to receive(:expire).with(:rendered_readme, :branch_names)
-      expect(redis_hash_cache).to receive(:delete).with(:rendered_readme, :branch_names)
+      expect(redis_set_cache).to receive(:expire).with(:branch_names)
+      expect(redis_hash_cache).to receive(:delete).with(:branch_names)
 
-      repository.expire_method_caches(%i(rendered_readme branch_names))
+      repository.expire_method_caches(%i(branch_names))
     end
 
     it 'does not expire caches for non-existent methods' do

spec/models/readme_blob_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe ReadmeBlob do
-  include FakeBlobHelpers
-
-  describe 'policy' do
-    let(:project) { build(:project, :repository) }
-
-    subject { described_class.new(fake_blob(path: 'README.md'), project.repository) }
-
-    it 'works with policy' do
-      expect(Ability.allowed?(project.creator, :read_blob, subject)).to be_truthy
-    end
-  end
-end

spec/models/repository_spec.rb

@@ -483,12 +483,6 @@ RSpec.describe Repository do
       it { is_expected.to be_an_instance_of(::Blob) }
     end
 
-    context 'readme blob on HEAD' do
-      subject { repository.blob_at(repository.head_commit.sha, 'README.md') }
-
-      it { is_expected.to be_an_instance_of(::ReadmeBlob) }
-    end
-
     context 'readme blob not on HEAD' do
       subject { repository.blob_at(repository.find_branch('feature').target, 'README.md') }
 
@@ -1938,7 +1932,6 @@ RSpec.describe Repository do
       expect(repository).to receive(:expire_method_caches).with([
         :size,
         :commit_count,
-        :rendered_readme,
         :readme_path,
         :contribution_guide,
         :changelog,
@@ -2314,14 +2307,6 @@ RSpec.describe Repository do
           expect(repository.readme).to be_nil
         end
       end
-
-      context 'when a README exists' do
-        let(:project) { create(:project, :repository) }
-
-        it 'returns the README' do
-          expect(repository.readme).to be_an_instance_of(ReadmeBlob)
-        end
-      end
     end
   end
 
@@ -2527,9 +2512,8 @@ RSpec.describe Repository do
   describe '#refresh_method_caches' do
     it 'refreshes the caches of the given types' do
       expect(repository).to receive(:expire_method_caches)
-        .with(%i(rendered_readme readme_path license_blob license_key license))
+        .with(%i(readme_path license_blob license_key license))
 
-      expect(repository).to receive(:rendered_readme)
       expect(repository).to receive(:readme_path)
       expect(repository).to receive(:license_blob)
       expect(repository).to receive(:license_key)