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
I'm accustomed to the PEP8 79 chars line limit (this is really helpful when using multiple columns for instance in Sublime Text). I know that django does not strictly follow this rule. On the other hand, the code is still readable IMO, so I would leave it as it is now.