SUNRPC: Cleanup to remove xs_tcp_close()

xs_tcp_close() is now just a call to xs_tcp_shutdown(), so remove it,
and replace the entry in xs_tcp_ops.

Suggested-by: Anna Schumaker <anna.schumaker@netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>