scene: inline subsurface_tree_destroy
scene: inline subsurface_tree_destroy

This is only called from one function.

To destroy the wlr_scene_subsurface_tree from elsewhere, callers
can destroy the scene-graph node returned by
wlr_scene_subsurface_tree_create instead (just like a compositor
would do). subsurface_tree_handle_surface_destroy does exactly this.

Inlining avoids calling subsurface_tree_destroy by mistake.

swaywm/wlroots,SirCmpwn/wlroots,swaywm/wlroots