handler_purge_permissions now call module manager

Update handler_purge_permissions to call the purge_obsolete model
manager method instead of the management command.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/permissions/handlers.py

@@ -1,7 +1,11 @@
 from __future__ import unicode_literals
 
-from django.core import management
+from django.apps import apps
 
 
 def handler_purge_permissions(**kwargs):
-    management.call_command(command_name='purgepermissions')
+    StoredPermission = apps.get_model(
+        app_label='permissions', model_name='StoredPermission'
+    )
+
+    StoredPermission.objects.purge_obsolete()