fix filter not working in [disable] (#2826)

The problem was that the specials_context was not set during those
two get_special_bool calls

This removes what seems to be a rather useless optimisation in
battle_context::battle_context since choose_attacker_weapon would
just return 0 in that case.

fixes #2801

(cherry-picked from commit c4cf0c9)

src/actions/attack.cpp

@@ -366,14 +366,6 @@ battle_context::battle_context(const unit_map& units,
 	const unit& defender = *units.find(defender_loc);
 	const double harm_weight = 1.0 - aggression;
 
-	if(attacker_weapon == -1 &&
-	   attacker.attacks().size() == 1 &&
-	   attacker.attacks()[0].attack_weight() > 0 &&
-	  !attacker.attacks()[0].get_special_bool("disable", true)
-	) {
-		attacker_weapon = 0;
-	}
-
 	if(attacker_weapon == -1) {
 		attacker_weapon = choose_attacker_weapon(
 			attacker, defender, units, attacker_loc, defender_loc, harm_weight, &defender_weapon, prev_def

src/gui/dialogs/unit_attack.cpp

@@ -122,7 +122,7 @@ void unit_attack::pre_show(window& window)
 			*defender.weapon : *no_weapon;
 
 		// Don't show if the atacker's weapon has at least one active "disable" special.
-		if(attacker_weapon.get_special_bool("disable")) {
+		if(attacker.disable) {
 			continue;
 		}