wayland/client: Free GSubprocessLauncher after spawning

A Meta.WaylandClient() object has a GSubprocessLauncher object
passed externally. Currently this object is kept while the
WaylandClient object exists, but is is only needed until the call
to spawn is made.

This patch frees that GSubprocessLauncher just after that call,
thus freeing those resources.

Fix https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1462