Django: Removing unique constraint and creating migration
I find the django automigration file generation problematic and incomplete. 
Actually I experienced another similar problem with django migration just yesterday. 
How I solved it:

delete all migration files from the /migrations folder
do a fresh makemigrations
run python manage.py migrate --fake so django does not try to rebuild.. 
Hey presto! Working and models updated :D
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.