fix(models): remove app_name from abstract models

When a subclass doesn't overrider Meta, it inherits the app_name from
the abstract class what is not descired behavior as it lands migrations
into social_django instead of the actuall app.

Fixes python-social-auth#496

Author: nijel

social_django/models.py

@@ -42,7 +42,6 @@ def __str__(self):
         return str(self.user)
 
     class Meta:
-        app_label = "social_django"
         abstract = True
 
     @classmethod