Remove custom merge strategy for db/schema.rb

This removes the custom merge strategy for db/schema.rb in case this is not needed anymore.

If I'm not mistaken, this basically uses https://gitlab.com/gitlab-org/merge-train/blob/master/bin/merge-db-schema to perform a 3-way merge for db/schema.rb.

For any conflicts other than the schema version, it uses the -ours conflict resolution. I tend to think this may lead to actually lose schema changes if those are conflicting with master. This is particularly concerning because the conflict detection works on hunks, not on individual lines. See merge-train!19 (comment 186554252).

This is a follow-up to #12434 (comment 185596118). A related MR was merge-train!19 (closed).