Windows: remove out of date cm_buf #define

Change-Id: Id60f509d7c8a1e9eb2f94d4c481d00e9b820b827
Reviewed-on: http://gerrit.openafs.org/8967
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>