This was pointlessly being converted between atom and string and back,
which with the switch from gdk_x11_get_xatom_name() to XGetAtomName()
also introduced a leak for every XGetAtomName() call.
Fixes: e66f4396e ("x11: Avoid GDK API in X11 selections")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2957>