Fix failing MySQL spec due to deadlock condition

`spec/features/uploads/user_uploads_file_to_note_spec.rb` was failing in
master because MySQL detected a deadlock when a DELETE and INSERT for
the same indexed item occurred within a transaction in the `uploads`
table.  Due to InnoDB's next-key locking algorithm
(innodb_locks_unsafe_for_binlog in
https://dev.mysql.com/doc/refman/5.5/en/innodb-parameters.html), InnoDB
sets an exclusive lock for any of the indexed records it encounters, so
the INSERT will fail until the DELETE is committed.

To fix this, we just disable the transaction for MySQL and keep
it for PostgreSQL.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/55161

app/uploaders/records_uploads.rb

@@ -23,13 +23,23 @@ module RecordsUploads
       return unless model
       return unless file && file.exists?
 
-      Upload.transaction do
+      # MySQL InnoDB may encounter a deadlock if a deletion and an
+      # insert is in the same transaction due to its next-key locking
+      # algorithm, so we need to skip the transaction.
+      # https://gitlab.com/gitlab-org/gitlab-ce/issues/55161#note_131556351
+      if Gitlab::Database.mysql?
+        readd_upload
+      else
+        Upload.transaction { readd_upload }
+      end
+    end
+
+    def readd_upload
       uploads.where(path: upload_path).delete_all
       upload.delete if upload
 
       self.upload = build_upload.tap(&:save!)
     end
-    end
     # rubocop: enable CodeReuse/ActiveRecord
 
     def upload_path