Fixes 'Build Mode' layering (#16548)

* Build Mode HUD plane

* `HUD_PLANE_BUILDMODE` moving

Also deleted `HUD_LAYER_SCREEN` since it's not used anywhere

code/__DEFINES/layers.dm

@@ -118,5 +118,8 @@
 #define SPLASHSCREEN_LAYER 23
 #define SPLASHSCREEN_PLANE 23
 
+// This should always be on top.
+#define HUD_PLANE_BUILDMODE 30
+
 ///Plane master controller keys
 #define PLANE_MASTERS_GAME "plane_masters_game"

code/_onclick/hud/_defines.dm

@@ -176,8 +176,3 @@
 //1 = standard hud
 //2 = reduced hud (just hands and intent switcher)
 //3 = no hud (for screenshots)
-
-
-#define HUD_LAYER_SCREEN 20
-
-#define HUD_LAYER_BUILDMODE 30

code/modules/buildmode/buttons.dm

@@ -1,7 +1,7 @@
 /obj/screen/buildmode
 	icon = 'icons/misc/buildmode.dmi'
 	var/datum/click_intercept/buildmode/bd
-	layer = HUD_LAYER_BUILDMODE
+	plane = HUD_PLANE_BUILDMODE
 
 /obj/screen/buildmode/New(bld)
 	bd = bld