volser: Don't assign code when we don't care

When we're cleaning up temporary volumes, we don't care whether
it succeeds or not. Don't assign code to the results of these
volume deletions, only to then discard it.

Caught by clang-analyzer

Change-Id: I81583cc82e78db38de8aad92c7c7d0c2f44eb33d
Reviewed-on: http://gerrit.openafs.org/9200
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>