xwayland: Cleanup and remove `xwayland.h`

Now that each source and header should be in order, we can safely cleaup
the last remaining bits from the main `xwayland.h` which is not needed
anymore and can be removed.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
3 jobs for xwayland-refactoring in 5 minutes and 23 seconds (queued for 1 second)
Status Job ID Name Coverage
  Docker Image
passed #1214737
debian-testing

00:00:31

 
  Build And Test
passed #1214738
autotools-build-and-test

00:04:52

passed #1214739
meson-build-and-test

00:02:46