volser: Don't catch errors in cleanup

We can't do anything if cleaning up the clone volume fails, so
don't bother storing the error code.

Caught by clang-analyzer

Change-Id: I5249dbad9a1bb732aba73e7a09f74c83e606b45d
Reviewed-on: http://gerrit.openafs.org/9258
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>