auth: Remove unecessary assignment

When we're counting the number of keys available, we don't need to
actually extract each key entry - remove the unused assignment.

Caught by clang-analyzer.

Change-Id: I98a30afccaf9a455ea0a7e77e7ca0d648abe4e70
Reviewed-on: http://gerrit.openafs.org/9145
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>