(LND_AMM): Became weird in 4.2.3: ammo use depends on l_dam if l_ammo

(LND_AMM): Became weird in 4.2.3: ammo use depends on l_dam if l_ammo
is non-zero; actual value of non-zero l_ammo was unimportant.  Side
effect: it substantially increased ammunition consumption at high
tech, because the change made the value increase with tech.  Back out
4.2.3's change, just use l_ammo.  This lets deities customize
ammunition consumption independent of damage.  Remove second
parameter.  Callers changed.