Remove the similarity_search feature flag

This change removes the similarity_search feature flag, the flag was
default enabled since 13.3

Changelog: removed

config/feature_flags/development/similarity_search.yml

----
-name: similarity_search
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/37300/
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/38675
-milestone: '13.3'
-type: development
-group: group::optimize
-default_enabled: true

doc/api/groups.md

@@ -288,11 +288,8 @@ Parameters:
 | `with_security_reports`       | boolean        | no       | **(ULTIMATE)** Return only projects that have security reports artifacts present in any of their builds. This means "projects with security reports enabled". Default is `false` |
 
 1. Order by similarity: Orders the results by a similarity score calculated from the provided `search`
-URL parameter. This is an [alpha](https://about.gitlab.com/handbook/product/gitlab-the-product/#alpha) feature [introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/221043) in GitLab 13.3.
-
-   The feature is behind a feature flag, you can [enable it](../administration/feature_flags.md#enable-or-disable-the-feature)
-with the `similarity_search` flag. When using `order_by=similarity` the `sort` parameter is
-ignored. When the `search` parameter is not provided, the API returns the projects ordered by `name`.
+URL parameter. When using `order_by=similarity`, the `sort` parameter is ignored. When the `search`
+parameter is not provided, the API returns the projects ordered by `name`.
 
 Example response:
 

lib/api/groups.rb

@@ -137,7 +137,7 @@ module API
 
       # rubocop: disable CodeReuse/ActiveRecord
       def handle_similarity_order(group, projects)
-        if params[:search].present? && Feature.enabled?(:similarity_search, group, default_enabled: true)
+        if params[:search].present?
           projects.sorted_by_similarity_desc(params[:search])
         else
           order_options = { name: :asc }

spec/requests/api/groups_spec.rb

@@ -965,23 +965,6 @@ RSpec.describe API::Groups do
             expect(project_names).to eq(['Project', 'Test', 'Test Project'])
           end
         end
-
-        context 'when `similarity_search` feature flag is off' do
-          before do
-            stub_feature_flags(similarity_search: false)
-          end
-
-          it 'returns items ordered by name' do
-            subject
-
-            expect(response).to have_gitlab_http_status(:ok)
-            expect(response).to include_pagination_headers
-            expect(json_response.length).to eq(2)
-
-            project_names = json_response.map { |proj| proj['name'] }
-            expect(project_names).to eq(['Test', 'Test Project'])
-          end
-        end
       end
 
       it "returns the group's projects with simple representation" do