Remove unused composer rake task

Delete the composer rake task related to composer
cache files which is an unused area of code.

lib/tasks/gitlab/packages/composer.rake

-# frozen_string_literal: true
-
-require 'logger'
-
-desc "GitLab | Packages | Build composer cache"
-namespace :gitlab do
-  namespace :packages do
-    task build_composer_cache: :environment do
-      logger = Logger.new($stdout)
-      logger.info('Starting to build composer cache files')
-
-      ::Packages::Package.composer.find_in_batches do |packages|
-        packages.group_by { |pkg| [pkg.project_id, pkg.name] }.each do |(project_id, name), packages|
-          logger.info("Building cache for #{project_id} -> #{name}")
-          Gitlab::Composer::Cache.new(project: packages.first.project, name: name).execute
-        end
-      end
-    end
-  end
-end

spec/tasks/gitlab/packages/composer_rake_spec.rb

-# frozen_string_literal: true
-
-require 'rake_helper'
-
-RSpec.describe 'gitlab:packages:build_composer_cache namespace rake task', :silence_stdout do
-  let_it_be(:package_name) { 'sample-project' }
-  let_it_be(:package_name2) { 'sample-project2' }
-  let_it_be(:json) { { 'name' => package_name } }
-  let_it_be(:json2) { { 'name' => package_name2 } }
-  let_it_be(:group) { create(:group) }
-  let_it_be(:project) { create(:project, :custom_repo, files: { 'composer.json' => json.to_json }, group: group) }
-  let_it_be(:project2) { create(:project, :custom_repo, files: { 'composer.json' => json2.to_json }, group: group) }
-
-  let!(:package) { create(:composer_package, :with_metadatum, project: project, name: package_name, version: '1.0.0', json: json) }
-  let!(:package2) { create(:composer_package, :with_metadatum, project: project, name: package_name, version: '2.0.0', json: json) }
-  let!(:package3) { create(:composer_package, :with_metadatum, project: project2, name: package_name2, version: '3.0.0', json: json2) }
-
-  before :all do
-    Rake.application.rake_require 'tasks/gitlab/packages/composer'
-  end
-
-  subject do
-    run_rake_task("gitlab:packages:build_composer_cache")
-  end
-
-  it 'generates the cache files' do
-    expect { subject }.to change { Packages::Composer::CacheFile.count }.by(2)
-  end
-end