RCONInstance: Remove pointless code that makes pthreads segfault

pthreads doesn't play well with resources, and this code upsets it. Also, this code is utterly pointless.

The whole of RCON needs burning to the ground...

src/pocketmine/network/rcon/RCONInstance.php

@@ -193,8 +193,6 @@ class RCONInstance extends Thread{
 				}
 			}
 		}
-		unset($this->socket, $this->cmd, $this->response, $this->stop);
-		exit(0);
 	}
 
 	public function getThreadName() : string{