layer-shell: stop sending configure events on surface creation

layer-shell: stop sending configure events on surface creation

With wlroots 0.18, layer-shell's new_surface event is emitted on
zwlr_layer_shell_v1.get_layer_surface request rather than the first
commit. This change layer-shell clients like fuzzel to flicker on launch
because labwc was sending a configure event with fullscreen geometry due
to the absence of geometry hints on get_layer_surface requests.

This commit removes the code that updates the usable area from
new_surface handler, preventing unintended configure events with
fullscreen geometry.