Remove handled knocker from the waiting participant list

[nandito]

We accidentally removed the waiting_participant_left event listener during a refactor (#68). This caused a bug in the knock handling: whenever a knock got accepted or rejected, it didn't get removed from the waiting participant list.

Test plan

1. Use a locked room in your sample app
2. Join with a host who can see and accept/reject waiting participants
3. Knock the room with a guest => host should see the knocker
4. Accept the knocker => it should join the room and the waiting participant list should be empty again

[kevinwhereby]

noice!