Convert a bunch of legacy overlay calls to SSoverlays (#4015)

Bunch of overlay conversions I found in an old stash I had forgotten about.

code/modules/mining/coins.dm

@@ -51,7 +51,7 @@
 			user << "<span class='notice'>There already is a string attached to this coin.</span>"
 			return
 		if (CC.use(1))
-			overlays += image('icons/obj/items.dmi',"coin_string_overlay")
+			add_overlay("coin_string_overlay")
 			string_attached = 1
 			user << "<span class='notice'>You attach a string to the coin.</span>"
 		else
@@ -65,7 +65,7 @@
 		var/obj/item/stack/cable_coil/CC = new/obj/item/stack/cable_coil(user.loc)
 		CC.amount = 1
 		CC.update_icon()
-		overlays = list()
+		cut_overlays()
 		string_attached = null
 		user << "<span class='notice'>You detach the string from the coin.</span>"
 	else ..()

code/modules/mining/mine_items.dm

@@ -593,7 +593,7 @@
 	cell = new /obj/item/weapon/cell/high(src)
 	key = null
 	var/image/I = new(icon = 'icons/obj/cart.dmi', icon_state = "[icon_state]_overlay", layer = src.layer + 0.2) //over mobs
-	overlays += I
+	add_overlay(I)
 	turn_off()	//so engine verbs are correctly set
 
 /obj/vehicle/train/cargo/engine/attackby(obj/item/weapon/W as obj, mob/user as mob)
@@ -1282,7 +1282,7 @@ var/list/total_extraction_beacons = list()
 		user.loc = src.loc
 		var/image/W = image('icons/obj/mining.dmi',"fitnessweight-w")
 		W.layer = 5.1
-		overlays += W
+		add_overlay(W)
 		var/bragmessage = pick("pushing it to the limit","going into overdrive","burning with determination","rising up to the challenge", "getting strong now","getting ripped")
 		user.visible_message("<B>[user] is [bragmessage]!</B>")
 		var/reps = 0
@@ -1305,7 +1305,7 @@ var/list/total_extraction_beacons = list()
 		animate(user, pixel_y = 0, time = 3)
 		var/finishmessage = pick("You feel stronger!","You feel like you can take on the world!","You feel robust!","You feel indestructible!")
 		icon_state = "fitnessweight"
-		overlays -= W
+		cut_overlay(W)
 		user << "[finishmessage]"
 		user.nutrition = user.nutrition - 10