make sniper not target buildings, vehicles

Sniper currently shoots at buildings when auto-attacking. This seems kind of wrong, given that he can blow them up with a C4 charge.

Sniper is probably also wasting his ammo by shooting at vehicles when the enemy is a mix of vehicles + infantry. Since the Sniper costs $1000, you would want him to kill some infantry before dying, to get your money's worth.

Part of the reason proposed change is that sniper is not useful when fighting a mix of vehicles + infantry (e.g. end-game light tanks + chem warriors). Sniper might be helpful in a mix, but currently wastes a lot of its ammo on vehicles, making it definitely not worth the money.