xen: Drop un-informative message during boot

On systems that are not booted as a Xen domain, the xenfs driver prints
the following message during boot.

[    3.460595] xenfs: not registering filesystem on non-xen platform

As the user chose not to boot a Xen domain, this message does not
provide useful information. Drop this message.

Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>