func_breakable shouldn't take damage when it's set to Trigger Only

[FreeSlave]

When func_breakable is set to Trigger Only, it sets the pev->takedamage to DAMAGE_NO, but since there's no check for it in TakeDamage it still can take damage because some attacks don't check for pev->takedamage from outside (e.g. CheckTraceHullAttack).

I'm not sure if this fix should be merged as there could be some maps that rely on such behavior.