s390/mm: Remove sev_active() function

All references to sev_active() were moved to arch/x86 so we don't need to
define it for s390 anymore.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Halil Pasic <pasic@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190806044919.10622-7-bauerman@linux.ibm.com