nfsd: silence rpcb_unset noise for NFSv4 only servers

An NFSv4 only configuration does not register with
rpcbind(). Without this patch a failure to rpcb_unset()
is reported when the daemon is terminated for this case.

This is harmless noise, but this patch avoids calling
rpcb_unset() for the NFSv4 only case, avoiding the noise.

When called with "-d", it still does the rpcb_unset(),
assuming that the configuration might have been
changed to NFSv4 only and unregistering with
rpcbind() might still be needed.

Reviewed by: freqlabs
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29449