s390/uv: Fix comment of uv_find_secret() function

The uv_get_secret_metadata() function has been removed some
months ago, so we should not mention it in the comment anymore.

Fixes: a42831f0 ("s390/uv: Remove uv_get_secret_metadata function")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>