s390/percpu: remove this_cpu_xor() implementation

this_cpu_xor() will be removed tree wide during the next merge window.
To avoid merge conflicts s390's removal comes via the s390 tree.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>