s390: Remove 2k vs 4k page table leftovers

Since commit d08d4e7c ("s390/mm: use full 4KB page for 2KB PTE") always
4k page tables are allocated, however there is still some (now) obsolete
code left which deals with switching from 2k to 4k page tables for qemu/kvm
processes.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Remove the not needed code.

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