s390/uv: Remove uv_get_secret_metadata function

s390/uv: Remove uv_get_secret_metadata function

The uv_get_secret_metadata() in-kernel function was only
offered and used by the pkey uv handler. Remove it as there
is no customer any more.

Suggested-by: Steffen Eiden <seiden@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Acked-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-24-freude@linux.ibm.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>