disable undoing unit creation.

Undoing unit recruits currently ends in oos in most cases, because the random generator is called during unit creation for creating random names.
This is not impossible to fix, ofc we could just enable it for nameless units, but i'm not 100% sure wether units with no names call the random generator for random names or not. Also names normaly don't have any influence on the game (same for genders maybe?), so there is no reason not to be able to undo it from a players point of view. We also cannot just store the random state in undo_stack->add_recruit, because the unit creation happens before that