volser: DoVolDelete returning VNOVOL is success

volser: DoVolDelete returning VNOVOL is success

When moving, copying or releasing volumes, do not treat a failure
to delete a volume because the volume no longer exists as an error.
The volume clone has flags

  VTDeleteOnSalvage | VTOutOfService

assigned to it which means that the fileserver won't attach the volume
and volume has its deleteMe field assigned the value of DESTROY_ME.
Such a volume will be deleted the next time the salvager scans the
partition.  Once the transaction is complete the volume might be
removed.

Change-Id: I0bd38906e3836e0c96f3784a8bd9ad63f5b857c6
Reviewed-on: https://gerrit.openafs.org/12976
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>