MINOR: session: small cleanup of conn_complete_session()

Commit 8e3c6ce ("MEDIUM: connection: get rid of data->init() which was
not for data") simplified conn_complete_session() but introduced a
confusing check which cannot happen on CO_FL_HANDSHAKE. Make it clear
that this call is final and will either succeed and complete the
session or fail.
diff --git a/src/session.c b/src/session.c
index 22f6b58..c4c9294 100644
--- a/src/session.c
+++ b/src/session.c
@@ -404,14 +404,12 @@
 {
 	struct session *sess = conn->owner;
 
+	/* the embryonic session's task is not needed anymore */
 	conn_clear_xprt_done_cb(conn);
 
 	if (conn->flags & CO_FL_ERROR)
 		goto fail;
 
-	if (conn->flags & CO_FL_HANDSHAKE)
-		return 0; /* wait more */
-
 	/* if logs require transport layer information, note it on the connection */
 	if (sess->fe->to_log & LW_XPRT)
 		conn->flags |= CO_FL_XPRT_TRACKED;