Remove validate_merge_sha feature flag

This removes the `validate_merge_sha` feature flag that was added in
https://gitlab.com/gitlab-org/gitlab/merge_requests/19579 which was
included in 12.5

Starting 12.5 we started writing the merge_sha inside the merge params
for validating.

To avoid merges failing during a deploy, the feature flag wasn't
defaulted to "on". But one release later we can remove it since the
SHAs will would have been written in the params regardless.

app/services/merge_requests/merge_service.rb

@@ -62,8 +62,6 @@ module MergeRequests
     end
 
     def updated_check!
-      return unless Feature.enabled?(:validate_merge_sha, merge_request.target_project, default_enabled: false)
-
       unless source_matches?
         raise_error('Branch has been updated since the merge was requested. '\
                     'Please review the changes.')

changelogs/unreleased/bvl-remove-validate-merge-sha-ff.yml

+---
+title: Validate the merge sha before merging, confirming that the merge will only
+  contain what the user saw
+merge_request: 20348
+author:
+type: fixed

spec/services/merge_requests/merge_service_spec.rb

@@ -104,14 +104,6 @@ describe MergeRequests::MergeService do
           .to change { merge_request.merge_error }
                 .from(nil).to(merge_error)
       end
-
-      it 'merges the MR when the feature is disabled' do
-        stub_feature_flags(validate_merge_sha: false)
-
-        service.execute(merge_request)
-
-        expect(merge_request).to be_merged
-      end
     end
 
     context 'closes related issues' do