Fix "Mentions & keywords" notification level

Make it work in cases where multiple accounts are present in the room
and one of them is highlighted.

src/backend/backend.py

@@ -8,7 +8,7 @@ import sys
 import time
 from datetime import datetime
 from pathlib import Path
-from typing import Any, DefaultDict, Dict, List, Optional, Set, Tuple, Union
+from typing import Any, DefaultDict, Dict, List, Optional, Tuple, Union
 from urllib.parse import urlparse
 
 import aiohttp
@@ -133,7 +133,6 @@ class Backend:
 
         self.mxc_events: DefaultDict[str, List[Event]] = DefaultDict(list)
 
-        self.notified_events:           Set[str]        = set()  # {event_id}
         self.notification_avatar_cache: Dict[str, Path] = {}     # {mxc: path}