Fix check whether a non-owned index is being dropped in DatabaseTableChangeProcessor

Fix check whether a non-owned index is being dropped in DatabaseTableChangeProcessor

The reproducer and fix is effectively identical to the one in
d7f721d6f920d66f75102723b504d89e57a8c9ff.

Package A: Installs KEY someIndex (`UNIQUE`)
Package B: Installs UNIQUE KEY someIndex2 (`UNIQUE`)
Package B: Drops UNIQUE KEY someIndex2 (`UNIQUE`)

It was erroneously detected that Package B would drop the index owned by
Package A. The actual dropping logic was already correct, just the safety check
was incorrect.