Removed argument from kill function

I've removed the `$pid` argument from the `kill()` function, because I think that PocketMine will only have to kill itself (the PHP process) if it ever has to, and I'm not comfortable with PocketMine being able to -9 kill any process on my system.
If any of you guys disagree with me, please let me know, and post your arguments, I'm open to discussions :smile: