Fixed #23614 -- Changed the way the migration autodetector orders unique/index_together

Ticket: https://code.djangoproject.com/ticket/23614

This patch only solves the problem for one situation: removing a field. Renaming a field and keeping it in the `index/unique_together` option shouldn't be a problem, since `_generate_altered_foo_together` works on the renamed fields.
I wouldn't move it to a helper method, at least not as part of this commit.