datetime: Remove datetime D-Bus mechanism

Now that gnome-control-center uses systemd's date & time mechanism[1],
we don't need to ship our own mechanism for that purpose. This also
removes the last user of dbus-glib in gnome-settings-daemon [2].

[1]: https://bugzilla.gnome.org/show_bug.cgi?id=658352
[2]: https://bugzilla.gnome.org/show_bug.cgi?id=622878