Django: Removing unique constraint and creating migration
Good question. A one to one relationship implies that one record is associated with another record uniquely. Even though the unique constraint is removed(for one to one field) in the code explicitly, it won't be reflected in your DB. So it won't create any migration file. 
If you try the same thing for foreign constraint, it will work.
I have created a model and migrated in Django, with a unique key constraint for one of the field. Now am trying to remove the unique constraint and generate another migration file with the new change, but it says "Nothing seems to have changed".
I tried with the command 
python manage.py schemamigration  --auto
PS: I am using OnetoOne relationship for the field.