Clean up confusing use of def->own in move_in_land()

Clean up confusing use of def->own in move_in_land()

It passed def->own to lnd_sweep(), which looks like a bug.  But it's
actually player->cnum there, because take_def() already set def->own
to player->owner: take_def() first changes the owner of the attacked
sector by calling takeover(), then updates def->own from that in
att_get_combat().