Fixes layering issue with tree icons

I've discovered from using it on the tether map in Vorestation that it
being on layer **9** is really unneccessary and it just looks absurd
when layered over other things.

I don't think you actually use trees for anything right now, but
whenever you do, you'll want this to prevent them from looking dumb.

code/game/objects/structures/flora.dm

@@ -4,7 +4,6 @@
 	anchored = 1
 	density = 1
 	pixel_x = -16
-	layer = 9
 
 /obj/structure/flora/tree/pine
 	name = "pine tree"