Unsurprisingly, MobArena's monster handling is sub-optimal, possibly due to premature optimization. Before, when e.g. a Creeper boss would explode, it would not be removed from the bosses collection, and thus the ability thread would continue on running. This patch adds a tiny bit of overhead w.r.t. removing dead monsters, but it fixes the issue with Creeper bosses, and with a bit of luck, mcMMO will not cause issues anymore (yet to be tested).