Remove async_merge_request_check_mergeability feature flag

It's been up for a couple of weeks now on GitLab.com and also
released as part of 13.0.

app/models/merge_request.rb

@@ -877,7 +877,7 @@ class MergeRequest < ApplicationRecord
 
     check_service = MergeRequests::MergeabilityCheckService.new(self)
 
-    if async && Feature.enabled?(:async_merge_request_check_mergeability, project, default_enabled: true)
+    if async
       check_service.async_execute
     else
       check_service.execute(retry_lease: false)

changelogs/unreleased/216939-remove-async-mr-check-ff.yml

+---
+title: Remove async_merge_request_check_mergeability feature flag
+merge_request: 33917
+author:
+type: changed

doc/api/merge_requests.md

@@ -74,8 +74,7 @@ endpoint, set the `with_merge_status_recheck` parameter to `true` in the query.
 
 NOTE: **Note:**
 [Starting in GitLab 12.8](https://gitlab.com/gitlab-org/gitlab/-/issues/29984),
-when `async_merge_request_check_mergeability` feature flag is enabled, the
-mergeability (`merge_status`) of each merge request will be checked
+the mergeability (`merge_status`) of each merge request will be checked
 asynchronously when a request is made to this endpoint. Poll this API endpoint
 to get updated status. This affects the `has_conflicts` property as it is
 dependent on the `merge_status`. It'll return `false` unless `merge_status` is
@@ -555,8 +554,7 @@ Parameters:
 
 NOTE: **Note:**
 [Starting in GitLab 12.8](https://gitlab.com/gitlab-org/gitlab/-/issues/29984),
-when `async_merge_request_check_mergeability` feature flag is enabled, the
-mergeability (`merge_status`) of a merge request will be checked
+the mergeability (`merge_status`) of a merge request will be checked
 asynchronously when a request is made to this endpoint. Poll this API endpoint
 to get updated status. This affects the `has_conflicts` property as it is
 dependent on the `merge_status`. It'll return `false` unless `merge_status` is

spec/models/merge_request_spec.rb

@@ -2369,24 +2369,10 @@ describe MergeRequest do
       end
 
       context 'when async is true' do
-        context 'and async_merge_request_check_mergeability feature flag is enabled' do
-          it 'executes MergeabilityCheckService asynchronously' do
-            expect(mergeability_service).to receive(:async_execute)
+        it 'executes MergeabilityCheckService asynchronously' do
+          expect(mergeability_service).to receive(:async_execute)
 
-            subject.check_mergeability(async: true)
-          end
-        end
-
-        context 'and async_merge_request_check_mergeability feature flag is disabled' do
-          before do
-            stub_feature_flags(async_merge_request_check_mergeability: false)
-          end
-
-          it 'executes MergeabilityCheckService' do
-            expect(mergeability_service).to receive(:execute)
-
-            subject.check_mergeability(async: true)
-          end
+          subject.check_mergeability(async: true)
         end
       end
     end