`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