Clean up some code duplication.

This also meant that shapeshifters wouldn't cast even natural spells
if they were in a spellcasting form, though right now there don't seem
to be any polymorph targets with both real and natural spells.
Clean up some dead code.

Monsters only casted if you weren't around in very specific circumstances,
so all the special-casing didn't do very much (it slightly affected
The Unspeakable's casting sometimes, I guess).

Also, ball lightnings don't have spells anymore.