[S390] sysctl: get rid of dead declaration

Remove dead sysctl_userprocess_debug. The variable is
gone since we map our old userprocess_debug sysctl to
the common code show_unhandled_signals sysctl on s390.

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