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
Can you use `self.assertNumberMigrations()`, `self.assertOperationTypes()` and `self.assertOperationAttributes()` instead of things like `self.assertEqual(first_action.__class__.__name__, "AlterUniqueTogether")`, please. Have a look through at the top of the test case on which arguments they expect and how they work. They are also heavily used all over the place in the autodetector tests. (A overall cleanup is in #3564.)