Remove FLAG_INTERACTIVE.

This is simpler than FLAG_VERBOSE: everywhere we need to check it, we
have a Seat available, so we can just make it a Seat query method.

sshserver.c

@@ -121,6 +121,7 @@ static const SeatVtable server_seat_vt = {
     nullseat_stripctrl_new,
     nullseat_set_trust_status,
     nullseat_verbose_no,
+    nullseat_interactive_no,
 };
 
 static void server_socket_log(Plug *plug, int type, SockAddr *addr, int port,