Update voiceConnections on destroy

Instead of manually removing VoiceConnections from
InternalClient.voiceConnections, just integrate the functionality into
VoiceConnection.destroy()

src/Client/InternalClient.js

@@ -180,7 +180,6 @@ export default class InternalClient {
 			var leave = (connection) => {
 				return new Promise((resolve, reject) => {
 					connection.destroy();
-					this.voiceConnections.remove(connection);
 					resolve();
 				});
 			};
@@ -210,7 +209,6 @@ export default class InternalClient {
 			// preserve old functionality for non-bots
 			if (this.voiceConnections[0]) {
                 this.voiceConnections[0].destroy();
-                this.voiceConnections.remove(this.voiceConnections[0])
 			}
 			return Promise.resolve();
 		}

src/Voice/VoiceConnection.js

@@ -69,6 +69,7 @@ export default class VoiceConnection extends EventEmitter {
 				}
 			}
 		);
+		this.client.internal.voiceConnections.remove(this);
 	}
 
 	stopPlaying() {