s390/extmem: remove code for 31 bit addressing mode

All supported releases of z/VM allow 64 bit subcodes and addressing mode
for diag 0x64.

This patch removes a lot of code for handling 31 bit addressing mode and
old subcodes.

Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>