Remove workaround for MongoDB 6 in DatabaseFeatures.supports_atlas_search

Follow up to ce390e8.

Author: timgraham

django_mongodb_backend/features.py

@@ -601,9 +601,7 @@ def django_test_skips(self):
     def supports_atlas_search(self):
         """Does the server support Atlas search queries and search indexes?"""
         try:
-            # An existing collection must be used on MongoDB 6, otherwise
-            # the operation will not error when unsupported.
-            self.connection.get_collection("django_migrations").list_search_indexes()
+            self.connection.get_collection("__null").list_search_indexes()
         except OperationFailure:
             # It would be best to check the error message or error code to
             # avoid hiding some other exception, but the message/code varies