Remove an extra definition.

missing_events will either be defined later, or the method will exit early.

synapse/storage/databases/main/events_worker.py

@@ -805,7 +805,6 @@ async def get_missing_events_from_cache_or_db() -> Dict[
                 # the events have been redacted, and if so pulling the redaction event
                 # out of the database to check it.
                 #
-                missing_events = {}
                 try:
                     # Try to fetch from any external cache. We already checked the
                     # in-memory cache above.