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.
almost only styling. 2 Small changes though:

1. in the inner `if` I moved the `j > i` before `self.check_dependency()`
2. I added a comment on what `ops = ops[:1] ...` does