core: Remove META_LAYER_FULLSCREEN

This layer isn't really being used and in fact, it causes
meta_stack_get_default_focus_window() to return a fullscreen window
even if the naturally topmost window in the stack isn't a fullscreen
one.

Note that commit a3bf9b01 changed how
we choose the default focus window from the MRU to the topmost in the
stack.

https://bugzilla.gnome.org/show_bug.cgi?id=768221