Remove cached_api_merge_request_version FF

The FF has been enabled by default for a while
Now we can remove it

changelogs/unreleased/id-remove-cached-api-ff.yml

+---
+title: Remove cached_api_merge_request_version feature flag
+merge_request: 58670
+author:
+type: changed

config/feature_flags/development/cached_api_merge_request_version.yml

----
-name: cached_api_merge_request_version
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57568
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326093
-milestone: '13.11'
-type: development
-group: group::source code
-default_enabled: true

lib/api/merge_request_diffs.rb

@@ -45,11 +45,7 @@ module API
 
         merge_request = find_merge_request_with_access(params[:merge_request_iid])
 
-        if Feature.enabled?(:cached_api_merge_request_version, default_enabled: :yaml)
-          present_cached merge_request.merge_request_diffs.find(params[:version_id]), with: Entities::MergeRequestDiffFull, cache_context: nil
-        else
-          present merge_request.merge_request_diffs.find(params[:version_id]), with: Entities::MergeRequestDiffFull
-        end
+        present_cached merge_request.merge_request_diffs.find(params[:version_id]), with: Entities::MergeRequestDiffFull, cache_context: nil
       end
     end
   end

spec/requests/api/merge_request_diffs_spec.rb

@@ -82,18 +82,6 @@ RSpec.describe API::MergeRequestDiffs, 'MergeRequestDiffs' do
 
         expect(Rails.cache.fetch(merge_request_diff.cache_key)).to be_present
       end
-
-      context 'when cached_api_merge_request_version is disabled' do
-        before do
-          stub_feature_flags(cached_api_merge_request_version: false)
-        end
-
-        it 'is not performed' do
-          get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}/versions/#{merge_request_diff.id}", user)
-
-          expect(Rails.cache.fetch(merge_request_diff.cache_key)).to be_nil
-        end
-      end
     end
   end
 end