src: Cleanup event listeners on WebSocket connections (#3681)

* src: Cleanup event listeners on WebSocket connections
Should prevent #3641 from happening, as well as double connections on a shard

* typings: Forgot to add the method

src/client/websocket/WebSocketManager.js

@@ -245,6 +245,8 @@ class WebSocketManager extends EventEmitter {
       });
 
       shard.on(ShardEvents.DESTROYED, () => {
+        shard._cleanupConnection();
+
         this.debug('Shard was destroyed but no WebSocket connection was present! Reconnecting...', shard);
 
         this.client.emit(Events.SHARD_RECONNECTING, shard.id);