management: Remove obsolete update_permissions management command.

This was part of the old Django/guardian-based permissions system.

zerver/management/commands/update_permissions.py

@@ -1,15 +0,0 @@
-from __future__ import absolute_import
-
-from typing import Any
-
-from django.core.management.base import BaseCommand
-from django.apps import apps
-from django.contrib.auth.management import create_permissions
-
-class Command(BaseCommand):
-    help = "Sync newly created object permissions to the database"
-
-    def handle(self, *args, **options):
-        # type: (*Any, **Any) -> None
-        # From http://stackoverflow.com/a/11914435/90777
-        create_permissions(apps.get_app_config("zerver"), apps.get_models(), 2)