Don't use _cursor_to_dict in find_next_generated_user_id_localpart

synapse/storage/registration.py

@@ -460,14 +460,12 @@ class RegistrationStore(RegistrationWorkerStore,
         """
         def _find_next_generated_user_id(txn):
             txn.execute("SELECT name FROM users")
-            rows = self.cursor_to_dict(txn)
 
             regex = re.compile("^@(\d+):")
 
             found = set()
 
-            for r in rows:
-                user_id = r["name"]
+            for user_id, in txn:
                 match = regex.search(user_id)
                 if match:
                     found.add(int(match.group(1)))