Fix leaks from switch from gdk_x11_get_xatom_name() to XGetAtomName()

To quote from https://docs.gtk.org/gdk3-x11/func.x11_get_xatom_name.html:

unlike XAtomName() and gdk_atom_name(), the result doesn’t need to be freed

So previously the results were not freed by mutter, which has not been adjusted when XGetAtomName() was used instead in !2836 (merged).