Added MUTATOR_IS_ENABLED macro.
This is a clean way to check if mutator is enabled. I went through all REGISTER_MUTATOR instances and changed them to use new macro. I've also eliminated all direct uses of g_instagib
and g_overkill
. I didn't have time to go through all cvars of all mutators to see if other old mutator checks are still in the code. Maybe in separate MR.