Fixed #23794 - FieldDoesNotExist error in migration with deleted fields and unique_together constraint

The code in `check_dependency` is too specific. For instance the `any` function for `operation.unique_together=set([])` always returned `False` which caused the bug to happen. This PR contains following changes:
* added test case for this bug
* added simplified condition

https://code.djangoproject.com/ticket/23794
Hehe, if you don't mind, sure :) In that case two additional comments:

- Rename the test to `test_foo_together_remove_fk`
- Use `self.book_foo_together` instead of `self.book_unique`