models: Remove redundant Meta.permissions on Realm model.

This is dead code leftover from the old way of handling admin
permissions.

zerver/migrations/0308_remove_reduntant_realm_meta_permissions.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2.17 on 2020-12-20 14:01
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0307_rename_api_super_user_to_can_forge_sender'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='realm',
+            options={},
+        ),
+    ]

zerver/models.py

@@ -627,12 +627,6 @@ class Realm(models.Model):
     def presence_disabled(self) -> bool:
         return self.is_zephyr_mirror_realm
 
-    class Meta:
-        permissions = (
-            ('administer', "Administer a realm"),
-            ('api_super_user', "Can send messages as other users for mirroring"),
-        )
-
 def realm_post_delete_handler(sender: Any, **kwargs: Any) -> None:
     # This would be better as a functools.partial, but for some reason
     # Django doesn't call it even when it's registered as a post_delete handler.