Windows: afslogon is only an authentication provider

Windows: afslogon is only an authentication provider

afslogon.dll is just a Authentication Provider DLL.  It does not
provide network file system browse and mapping functions.  Therefore,
do not include the "ProviderPath" registry value when the
AuthentProviderPath variable is sufficient.

Change-Id: I92848e614906c3e35b882dd70570bddfaa44d36f
Reviewed-on: http://gerrit.openafs.org/7550
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>