[2.2] Remove redundant check for Meta.StackLayer.FULLSCREEN

Meta.StackLayer.FULLSCREEN is only set when metaWindow.is_fullscreen() is true, and even then not for all fullscreen windows.  See muffin's src/core/stack.c get_standalone_layer() for the conditions under which META_LAYER_FULLSCREEN is set.

This change is intended for after release of LM16.