CompressBatchedTask: remove useless code causing build failure

assuming that pthreads does not have leak issues, this code is pointless, and if pthreads does have leak issues, this may not prevent them anyway.

src/pocketmine/network/CompressBatchedTask.php

@@ -48,7 +48,6 @@ class CompressBatchedTask extends AsyncTask{
 	public function onRun(){
 		$batch = new BatchPacket();
 		$batch->payload = $this->data;
-		$this->data = null;
 
 		$batch->setCompressionLevel($this->level);
 		$batch->encode();