Windows: cm_DiscardSCache volumeCreationDate

Windows: cm_DiscardSCache volumeCreationDate

commit f716962ab41847af4450d0a361f5de9195b32ed0
clears the cm_scache_t volumeCreationDate field.
It shouldn't because the volumeCreationDate is not a property of
the callback.  It is a property of the status information which
does not change simply because the callback expires.

Change-Id: I3f73cf89588de6c1c87434cd04b86939d45cfa43
Reviewed-on: http://gerrit.openafs.org/7678
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>