Remove bitbucket_server_convert_mentions_to_users feature flag

What does this MR do and why?

Removes the bitbucket_server_convert_mentions_to_users feature flag that has been globally enabled on .com since 2024-01-31.

Rollout issue: [Feature flag] Rollout of `bitbucket_server_con... (#434453 - closed)

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How to set up and validate locally

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Relates to #434453 (closed)

app/workers/gitlab/bitbucket_server_import/stage/import_repository_worker.rb

@@ -14,11 +14,7 @@ def import(project)
 
           importer.execute
 
-          if Feature.enabled?(:bitbucket_server_convert_mentions_to_users, project.creator)
-            ImportUsersWorker.perform_async(project.id)
-          else
-            ImportPullRequestsWorker.perform_async(project.id)
-          end
+          ImportUsersWorker.perform_async(project.id)
         end
 
         def importer_class