s390/sysctl: Remove "vm/allocate_pgste" sysctl

Remove the not needed "vm/allocate_pgste" sysctl. It has no effect
anymore. However this is a user space visible change. It shouldn't cause
any problems, however if it does this needs to be partially reverted.

Note that some distributions set
vm/allocate_pgste=1

in one of the various sysctl configuration files. Besides a warning about
the (now) non-existent procfs file this doesn't cause any problems.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>