SUNRPC: Remove obsolete comment

Clean up.  The documenting comment at the top of net/sunrpc/clnt.c is
out of date.  We adopted BSD's RTO estimation mechanism years ago.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>