Remove retarget_merge_requests feature flag

This feature was implemented, tested, and rolled out.
This removes feature flag.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/320895

Changelog: changed

app/services/merge_requests/retarget_chain_service.rb

@@ -5,8 +5,6 @@ module MergeRequests
     MAX_RETARGET_MERGE_REQUESTS = 4
 
     def execute(merge_request)
-      return unless Feature.enabled?(:retarget_merge_requests, merge_request.target_project, default_enabled: :yaml)
-
       # we can only retarget MRs that are targeting the same project
       return unless merge_request.for_same_project? && merge_request.merged?
 

config/feature_flags/development/retarget_merge_requests.yml

----
-name: retarget_merge_requests
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/53710
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/320895
-milestone: '13.9'
-type: development
-group: group::memory
-default_enabled: true

doc/user/project/merge_requests/getting_started.md

@@ -172,11 +172,6 @@ is set for deletion, the merge request widget displays the
 > - [Disabled on self-managed](https://gitlab.com/gitlab-org/gitlab/-/issues/320902) in GitLab 13.9.
 > - [Enabled on self-managed](https://gitlab.com/gitlab-org/gitlab/-/issues/320895) GitLab 13.10.
 
-FLAG:
-On self-managed GitLab, by default this feature is available. To hide the feature,
-ask an administrator to
-[disable the `retarget_merge_requests` flag](../../../administration/feature_flags.md).
-
 In specific circumstances, GitLab can retarget the destination branch of
 open merge request, if the destination branch merges while the merge request is
 open. Merge requests are often chained in this manner, with one merge request

spec/services/merge_requests/retarget_chain_service_spec.rb

@@ -45,14 +45,6 @@ RSpec.describe MergeRequests::RetargetChainService do
             .from(merge_request.source_branch)
             .to(merge_request.target_branch)
         end
-
-        context 'when FF retarget_merge_requests is disabled' do
-          before do
-            stub_feature_flags(retarget_merge_requests: false)
-          end
-
-          include_examples 'does not retarget merge request'
-        end
       end
 
       context 'in the same project' do