clock: Use settings from gsettings-desktop-schemas

A key for 12hr/24hr clock format has been added to gsettings-desktop-schemas,
so use that instead of the one from the shell clock schema.

As the setting can be controlled from the Date and Time panel of
gnome-control-center now, drop the temporary preference dialog
as well.

https://bugzilla.gnome.org/show_bug.cgi?id=633200