kauth: Don't store the old signal routine

If we're not going to make use of the return from signal() don't
bother assigning it to a variable

Change-Id: I03cae7ab91d49e90f150a9dc4358c173920adc28
Reviewed-on: http://gerrit.openafs.org/9173
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>