Remove feature flag for squash commit persistence

Feature flag: https://gitlab.com/gitlab-org/gitlab/-/issues/294243

app/services/merge_requests/merge_service.rb

@@ -88,13 +88,9 @@ module MergeRequests
     end
 
     def try_merge
-      merge = repository.merge(current_user, source, merge_request, commit_message)
-
-      if merge_request.squash_on_merge? && Feature.enabled?(:persist_squash_commit_sha_for_squashes, project)
-        merge_request.update_column(:squash_commit_sha, source)
+      repository.merge(current_user, source, merge_request, commit_message).tap do
+        merge_request.update_column(:squash_commit_sha, source) if merge_request.squash_on_merge?
       end
-
-      merge
     rescue Gitlab::Git::PreReceiveError => e
       raise MergeError,
             "Something went wrong during merge pre-receive hook. #{e.message}".strip

changelogs/unreleased/21686_persist_squash_commit_sha.yml

+---
+title: Persist 'squash_commit_sha' when squashing
+merge_request: 51074
+author:
+type: added

config/feature_flags/development/persist_squash_commit_sha_for_squashes.yml

----
-name: persist_squash_commit_sha_for_squashes
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/50178
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/294243
-milestone: '13.8'
-type: development
-group: group::source code
-default_enabled: false

spec/services/merge_requests/merge_service_spec.rb

@@ -19,12 +19,8 @@ RSpec.describe MergeRequests::MergeService do
       { commit_message: 'Awesome message', sha: merge_request.diff_head_sha }
     end
 
-    let(:feature_flag_persist_squash) { true }
-
     context 'valid params' do
       before do
-        stub_feature_flags(persist_squash_commit_sha_for_squashes: feature_flag_persist_squash)
-
         allow(service).to receive(:execute_hooks)
         expect(merge_request).to receive(:update_and_mark_in_progress_merge_commit_sha).twice.and_call_original
 
@@ -90,14 +86,6 @@ RSpec.describe MergeRequests::MergeService do
 
           expect(merge_request.squash_commit_sha).to eq(squash_commit.id)
         end
-
-        context 'when feature flag is disabled' do
-          let(:feature_flag_persist_squash) { false }
-
-          it 'does not populate squash_commit_sha' do
-            expect(merge_request.squash_commit_sha).to be_nil
-          end
-        end
       end
     end