gdk/wayland/surface: Remove redundant `should_be_mapped` code
As far as I can tell, the code here is redundant and probably ended up
this way for historical reasons. A drag surface without
->is_drag_surface
would be created if gdk_display_create_surface
were called with GDK_SURFACE_TEMP
, but drag surfaces never seem to be
created that way.
In gtk4-demos
, drag and drop and popovers seem to be working normally
with this.