bosserver: Remove dead assignment in SaveOldFiles

We don't care about whether the renames succeed in SaveOldFiles,
so don't bother assigning their results to a variable

Caught by clang-analyzer

Change-Id: I0605fc71d243ef0f587706f9f3856aa4d722fb82
Reviewed-on: http://gerrit.openafs.org/9207
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>