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>
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