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

typings/index.d.ts

@@ -1717,6 +1717,7 @@ declare module 'discord.js' {
 		private _send(data: object): void;
 		private processQueue(): void;
 		private destroy(closeCode: number): void;
+		private _cleanupConnection(): void;
 
 		public send(data: object): void;
 		public on(event: 'ready', listener: () => void): this;