s390/protvirt: avoid memory sharing for diag 308 set/store

This reverts commit db9492ce ("s390/protvirt: add memory sharing for
diag 308 set/store") which due to ultravisor implementation change is
not needed after all.

Fixes: db9492ce ("s390/protvirt: add memory sharing for diag 308 set/store")
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>