Delete the unused "nd" argument for nfsrv_proxyds().

The "nd" argument for nfsrv_proxyds() is no longer used by the function.
This patch deletes it. This allows a subsequent patch to delete the "nd"
argument from nfsvno_getattr(), since it's only use of "nd" was to pass it
to nfsrv_proxyds().
Getting rid of the "nd" argument from nfsvno_getattr() avoids confusion
over why it might need "nd".

This patch is trivial and does not have any semantic effect.