DataPacket no longer keeps its own serializer

since a while ago, we're anyway just discarding the internal buffer anyway when the packet is repeatedly encoded, so this doesn't serve any advantage anymore.
We do need a system to be able to reuse encoded packet buffers, but right now we're not reusing them anyway.

tests/phpunit/network/mcpe/protocol/LoginPacketTest.php

@@ -43,6 +43,6 @@ class LoginPacketTest extends TestCase{
 		$pk = PacketPool::getInstance()->getPacket($stream->getBuffer());
 
 		$this->expectException(PacketDecodeException::class);
-		$pk->decode(); //bang
+		$pk->decode(new PacketSerializer($stream->getBuffer())); //bang
 	}
 }