Undead: remove an unused structure field.

Undead: remove an unused structure field.

I noticed that state->common, which is shared between all the game
states in an undo chain, has a 'solved' flag in it. That's not right -
solvedness as a property of a particular state on the chain belongs in
the game_state itself, and having-at-one-point-been-solved-ness as a
persistent property of the whole chain belongs in the game_ui.

Fortunately, the game isn't actually doing it wrong:
state->common->solved is set once and then never read, so it must have
been left in from early abandoned code. Now removed.