Removed unused model's Options.installed.

Unused since f9698c4391.

django/db/models/options.py

@@ -141,10 +141,6 @@ class Options:
         # Don't go through get_app_config to avoid triggering imports.
         return self.apps.app_configs.get(self.app_label)
 
-    @property
-    def installed(self):
-        return self.app_config is not None
-
     def contribute_to_class(self, cls, name):
         from django.db import connection
         from django.db.backends.utils import truncate_name