fire: Drop useless special case "shp_fire() returns zero"

Can't actually happen with the current damage formulas.  If it could,
then the special treatment would be inconsistent with sectors and land
units.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/lib/commands/mfir.c

@@ -283,7 +283,7 @@ multifire(void)
 		dam = shp_fire(&fship);
 	    }
 	    putship(fship.shp_uid, &fship);
-	    if (dam == 0 || CANT_HAPPEN(dam < 0)) {
+	    if (CANT_HAPPEN(dam < 0)) {
 		pr("Klick!     ...\n");
 		continue;
 	    }