Windows: remove CM_BUF_CMBKGFETCH flag

Windows: remove CM_BUF_CMBKGFETCH flag

The processing of the CM_BUF_CMBKGFETCH flag adds more
overhead than it prevents.  Get rid of it.

LICENSE MIT

Reviewed-on: http://gerrit.openafs.org/5359
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit b6f23864ffa380923f50d424063ad0804e407f4a)

Change-Id: I423aa105bad99c70942d6b967a1dde70216fb096
Reviewed-on: http://gerrit.openafs.org/6033
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>
Tested-by: Jeffrey Altman <jaltman@secure-endpoints.com>