remove ARCH_DEFAULT_KEXEC from Kconfig.kexec

remove ARCH_DEFAULT_KEXEC from Kconfig.kexec

This patch is a minor cleanup to the series "refactor Kconfig to
consolidate KEXEC and CRASH options".

In that series, a new option ARCH_DEFAULT_KEXEC was introduced in order to
obtain the equivalent behavior of s390 original Kconfig settings for
KEXEC.  As it turns out, this new option did not fully provide the
equivalent behavior, rather a "select KEXEC" did.

As such, the ARCH_DEFAULT_KEXEC is not needed anymore, so remove it.

Link: https://lkml.kernel.org/r/20230802161750.2215-1-eric.devolder@oracle.com
Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
Acked-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>