s390/alternatives: Remove unused empty header file

Remove the unused and empty arch/s390/kernel/alternative.h header
file which was added by mistake.

Fixes: 5ade5be4 ("s390: Add infrastructure to patch lowcore accesses")
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>