Remove callback for nio.UnknownBadEvent

Should this type of event appear, our function to register events won't
find the attributes required to process it that are present on normal
events.

src/backend/matrix_client.py

@@ -2357,7 +2357,7 @@ class MatrixClient(nio.AsyncClient):
     async def register_nio_event(
         self,
         room:                   nio.MatrixRoom,
-        ev:                     nio.Event,
+        ev:                     Union[nio.Event, nio.BadEvent],
         event_id:               str            = "",
         override_fetch_profile: Optional[bool] = None,
         **fields,

src/backend/nio_callbacks.py

@@ -675,13 +675,6 @@ class NioCallbacks:
         await self.client.register_nio_event(room, ev, content=co)
 
 
-    async def onUnknownBadEvent(
-        self, room: nio.MatrixRoom, ev: nio.UnknownBadEvent,
-    ) -> None:
-        co = "%1 sent a malformed event lacking a minimal structure"
-        await self.client.register_nio_event(room, ev, content=co)
-
-
     async def onUnknownEvent(
         self, room: nio.MatrixRoom, ev: nio.UnknownEvent,
     ) -> None: