sysupdated: Drop the Update() D-Bus method

sysupdated: Drop the Update() D-Bus method

Now that it’s been replaced by `Acquire()` and `Install()`, there’s no
need to expose it as a separate method. Wrappers around `sysupdated` can
still expose an ‘update’ convenience verb by calling those two methods.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>
Helps: https://github.com/systemd/systemd/issues/34814