Fix error in migrations

I was getting the following error when running 0035:
AttributeError: 'Edition' object has no attribute 'get_rank',

But that line seems to be unnecessary, since save() does the same thing,
and we are already calling that.

bookwyrm/migrations/0035_edition_edition_rank.py

@@ -8,7 +8,6 @@ def set_rank(app_registry, schema_editor):
     db_alias = schema_editor.connection.alias
     books = app_registry.get_model('bookwyrm', 'Edition')
     for book in books.objects.using(db_alias):
-        book.edition_rank = book.get_rank
         book.save()
 
 class Migration(migrations.Migration):