xen: remove a confusing comment on auto-translated guest I/O

xen: remove a confusing comment on auto-translated guest I/O

After removing the conditional return from xen_create_contiguous_region(),
the accompanying comment was left in place, but it now precedes an
unrelated conditional and confuses readers.

Fixes: 989513a735f5 ("xen: cleanup pvh leftovers from pv-only sources")
Signed-off-by: Petr Tesarik <petr.tesarik.ext@huawei.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20230802163151.1486-1-petrtesarik@huaweicloud.com
Signed-off-by: Juergen Gross <jgross@suse.com>