CLEANUP: connection: remove the definitions of conn_xprt_{stop,want}_{send,recv}

CLEANUP: connection: remove the definitions of conn_xprt_{stop,want}_{send,recv}

This marks the end of the transition from the connection polling states
introduced in 1.5-dev12 and the subscriptions in that arrived in 1.9.
The socket layer can now safely use its FD while all upper layers rely
exclusively on subscriptions. These old functions were removed. Some may
deserve some renaming to improved clarty though. The single call to
conn_xprt_stop_both() was dropped in favor of conn_cond_update_polling()
which already does the same.