selftests/livepatch: Remove unused local variable in set_ftrace_enabled()

set_ftrace_enabled() contains unused local variable "sysctl". Remove it.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>