CLEANUP: connection: remove unused CO_FL_WAIT_DATA

Very early in the connection rework process leading to v1.5-dev12, commit
56a77e5 ("MEDIUM: connection: complete the polling cleanups") marked the
end of use for this flag which since was never set anymore, but it continues
to be tested. Let's kill it now.
diff --git a/src/connection.c b/src/connection.c
index c23f464..f054cd1 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -109,7 +109,7 @@
 	}
 
 	if (conn->xprt && fd_send_ready(fd) &&
-	    ((conn->flags & (CO_FL_DATA_WR_ENA|CO_FL_WAIT_DATA|CO_FL_ERROR|CO_FL_HANDSHAKE)) == CO_FL_DATA_WR_ENA)) {
+	    ((conn->flags & (CO_FL_DATA_WR_ENA|CO_FL_ERROR|CO_FL_HANDSHAKE)) == CO_FL_DATA_WR_ENA)) {
 		/* force reporting of activity by clearing the previous flags :
 		 * we'll have at least ERROR or CONNECTED at the end of an I/O,
 		 * both of which will be detected below.