Stop landmines from changing the monster list mid-iteration

It looks like landmines would trigger game::explode_mon, which calls z.erase(). Unfortunately, landmines are frequently stepped on in the middle of game::monmove, causing issues with the iteration being inaccurate.

Removing that erase for now, since I honestly can't tell why it's necessary. Hopefully that doesn't break anything, or if it does, hopefully the broken thing is rather dramatic.