s390/kexec: remove CONFIG_KEXEC

Since "Kconfig: split the s390 base menu" CONFIG_KEXEC gets always selected.
Therefore there is no point in keeping CONFIG_KEXEC anywhere.

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