[MIRROR] Moves playsound_local() to mob and gets rid of a useless proc. (#710)

* Moves playsound_local() to mob and gets rid of a useless proc.

* Fixes compile errors

code/game/objects/effects/step_triggers.dm

@@ -183,8 +183,9 @@
 	if(!T)
 		return
 
-	if(triggerer_only)
-		A.playsound_local(T, sound, volume, freq_vary)
+	if(triggerer_only && ismob(A))
+		var/mob/B = A
+		B.playsound_local(T, sound, volume, freq_vary)
 	else
 		playsound(T, sound, volume, freq_vary, extra_range)