chore(signaling server): remove disconnect listener

Notes:
Note that it is not necessary to call socket.leave() during the disconnect event. This will happen automatically. Empty rooms will be automatically pruned so there is no need to manually remove them.
https://github.com/LearnBoost/socket.io/wiki/Rooms#joining-and-leaving
refactor(signaling server): rename the listener

Before:
- chat:joinRoom

After
- join_room