Do not show participants invisible to the user

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/347407

**Problem**

We expose participants that the current user cannot see because we
don't provide the current user as an argument to participants method
in GraphQL. When the user is missing, then we use the author of the
issuable permissions to fetch participants.

**Solution**

Remove the feature flag to enable verification for participants

Changelog: changed

app/models/concerns/participable.rb

@@ -64,8 +64,6 @@ module Participable
   #
   # Returns an Array of User instances.
   def visible_participants(user)
-    return participants(user) unless Feature.enabled?(:verify_participants_access, project, default_enabled: :yaml)
-
     filter_by_ability(raw_participants(user, verify_access: true))
   end
 

config/feature_flags/development/verify_participants_access.yml

----
-name: verify_participants_access
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/74906
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/347407
-milestone: '14.6'
-type: development
-group: group::source code
-default_enabled: false

spec/models/concerns/participable_spec.rb

@@ -138,7 +138,7 @@ RSpec.describe Participable do
       allow(instance).to receive_message_chain(:model_name, :element) { 'class' }
       expect(instance).to receive(:foo).and_return(user2)
       expect(instance).to receive(:bar).and_return(user3)
-      expect(instance).to receive(:project).thrice.and_return(project)
+      expect(instance).to receive(:project).twice.and_return(project)
 
       participants = instance.visible_participants(user1)
 
@@ -159,31 +159,10 @@ RSpec.describe Participable do
 
         allow(instance).to receive_message_chain(:model_name, :element) { 'class' }
         allow(instance).to receive(:bar).and_return(user2)
-        expect(instance).to receive(:project).thrice.and_return(project)
+        expect(instance).to receive(:project).twice.and_return(project)
 
         expect(instance.visible_participants(user1)).to be_empty
       end
-
-      context 'when feature flag is disabled' do
-        before do
-          stub_feature_flags(verify_participants_access: false)
-        end
-
-        it 'returns unavailable participants' do
-          model.participant(:bar)
-
-          instance = model.new
-          user1 = build(:user)
-          user2 = build(:user)
-          project = build(:project, :public)
-
-          allow(instance).to receive_message_chain(:model_name, :element) { 'class' }
-          allow(instance).to receive(:bar).and_return(user2)
-          expect(instance).to receive(:project).thrice.and_return(project)
-
-          expect(instance.visible_participants(user1)).to match_array([user2])
-        end
-      end
     end
   end