s390/debug: Remove s390dbf_sysctl_header variable

s390/debug: Remove s390dbf_sysctl_header variable

allmodconfig with clang W=1 points out an unused global variable:

arch/s390/kernel/debug.c:1237:33: error: variable
 's390dbf_sysctl_header' set but not used [-Werror,-Wunused-but-set-global]

Just remove the variable. There is no point in adding error handling for a
failing register_sysctl() call.

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>