xwayland: remove wl_log_set_handler_client workaround

This bumps the minimum Wayland version to 1.5 (released in 2014).

Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Martin Peres <martin.peres@mupuf.org>
4 jobs for remove-cast-workaround in 5 minutes and 52 seconds (queued for 1 second)