/deop <tab> no longer crashes large servers. Fixes BUKKIT-5742

/deop and then pressing tab used to crash large servers as it called the method Bukkit.getOfflinePlayers(). There is no reason as to why you would use getOfflinePlayers as you can only deop someone who is already op, so looking through every single player (possibly thousands) doesn't make much sense when you can just look through the operators instead.