Remove M_ACTUAL_SPELLS.

It didn't have many uses, anymore. One change -- all spells from before
the mon-spell-slots merge will be turned into wizard ones, which is not
ideal, but it's the simplest thing to do.
Remove M_PRIEST.

Again, not very necessary; this lets monsters without gods to
polymorph into priests, but that already had been happening for years
before 3f29927a2a fixed the behavior. It's not noticeable enough to
justify keeping the confusion for people reading the source.
Put the weight in mon-data.h for all monsters, not just ones with corpses.