xen: Remove some deadcode (x)

Remove three uncalled functions:

  xenbus_mkdir() was added in 2007 by
commit 4bac07c9 ("xen: add the Xenbus sysfs and virtual device hotplug
driver")
but has remained unused.

  xen_get_runstate_snapshot() last use was removed in 2016 by
commit 6ba286ad ("xen: support runqueue steal time on xen")
which replaces the use by the _cpu version.

  xen_resume_notifier_unregister() last use was removed in 2017 by
commit 1914f0cd ("xen/acpi: upload PM state from init-domain to Xen")

Remove them.

Signed-off-by: "Dr. David Alan Gilbert" <linux@treblig.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20250713132625.164728-1-linux@treblig.org>