removed unused BLANK_CHOICE_NONE

BLANK_CHOICE_NONE is defined, but not used anywhere, so I presume it can be got rid of.

I tried ``git rev-list --branches|xargs git grep BLANK_CHOICE_NONE | grep -v "django/db/models/fields/__init__.py"`` to search through the history for it in other locations, but that seems to crash after a couple of hundred thousand matches from the first part.