Revert "Fix dropping locks on shut down"

This reverts commit 83f1ccf.

synapse/storage/databases/main/lock.py

@@ -78,11 +78,7 @@ async def _on_shutdown(self) -> None:
         """Called when the server is shutting down"""
         logger.info("Dropping held locks due to shutdown")
 
-        # We need to take a copy of the tokens dict as dropping the locks will
-        # cause the dictionary to change.
-        tokens = dict(self._live_tokens)
-
-        for (lock_name, lock_key), token in tokens.items():
+        for (lock_name, lock_key), token in self._live_tokens.items():
             await self._drop_lock(lock_name, lock_key, token)
 
         logger.info("Dropped locks due to shutdown")