Deleting the related collection of `SequenceField` on field remove or change type

[bdragon300]

The related collection should be deleted if field of type SequenceField is deleted or changed type to smth another. Also keep in mind that the collection can be in different connection (SequenceField.db_alias). Also don't forget to check if the collection is already defined in Documents. If so, then do nothing.

Maybe it's better to interactively ask user if do the following during executing makemigrations command.