s390/mm: remove unused tprot() function

Since commit 461e0da7ddbb ("s390: remove broken hibernate / power
management support") there are no users of tprot() left. Remove
the function itself as well.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>