Remove the store_mentioned_users_to_db feature flag

Cleanup the enabled by default store_mentioned_users_to_db
development feature flag.

app/models/concerns/mentionable.rb

@@ -81,13 +81,6 @@ module Mentionable
   end
 
   def store_mentions!
-    # if store_mentioned_users_to_db feature flag is not enabled then consider storing operation as succeeded
-    # because we wrap this method in transaction with with_transaction_returning_status, and we need the status to be
-    # successful if mentionable.save is successful.
-    #
-    # This line will get removed when we remove the feature flag.
-    return true unless store_mentioned_users_to_db_enabled?
-
     refs = all_references(self.author)
 
     references = {}
@@ -253,15 +246,6 @@ module Mentionable
   def model_user_mention
     user_mentions.where(note_id: nil).first_or_initialize
   end
-
-  # We need this method to be checking that store_mentioned_users_to_db feature flag is enabled at the group level
-  # and not the project level as epics are defined at group level and we want to have epics store user mentions as well
-  # for the test period.
-  # During the test period the flag should be enabled at the group level.
-  def store_mentioned_users_to_db_enabled?
-    return Feature.enabled?(:store_mentioned_users_to_db, self.project&.group, default_enabled: true) if self.respond_to?(:project)
-    return Feature.enabled?(:store_mentioned_users_to_db, self.group, default_enabled: true) if self.respond_to?(:group)
-  end
 end
 
 Mentionable.prepend_if_ee('EE::Mentionable')

changelogs/unreleased/isolate-mentions-migration.yml

+---
+title: Store user mentions to DB
+merge_request: 43393
+author:
+type: added

config/feature_flags/development/store_mentioned_users_to_db.yml

----
-name: store_mentioned_users_to_db
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/19088
-rollout_issue_url: 
-group: group::project management
-type: development
-default_enabled: true

spec/support/shared_examples/models/mentionable_shared_examples.rb

@@ -207,29 +207,8 @@ RSpec.shared_examples 'an editable mentionable' do
 end
 
 RSpec.shared_examples 'mentions in description' do |mentionable_type|
-  describe 'when store_mentioned_users_to_db feature disabled' do
+  describe 'when storing user mentions' do
     before do
-      stub_feature_flags(store_mentioned_users_to_db: false)
-      mentionable.store_mentions!
-    end
-
-    context 'when mentionable description contains mentions' do
-      let(:user) { create(:user) }
-      let(:mentionable) { create(mentionable_type, description: "#{user.to_reference} some description") }
-
-      it 'stores no mentions' do
-        expect(mentionable.user_mentions.count).to eq 0
-      end
-
-      it 'renders description_html correctly' do
-        expect(mentionable.description_html).to include("<a href=\"/#{user.username}\" data-user=\"#{user.id}\"")
-      end
-    end
-  end
-
-  describe 'when store_mentioned_users_to_db feature enabled' do
-    before do
-      stub_feature_flags(store_mentioned_users_to_db: true)
       mentionable.store_mentions!
     end