CLEANUP: muxes/stream-int: Remove flags CS_FL_READ_NULL and SI_FL_READ_NULL

Since the flag CF_SHUTR is no more set to mark the end of the message, these
flags become useless.

This patch should be backported to 1.9.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 506226c..ab8504e 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -1443,7 +1443,7 @@
 		flags |= CS_FL_ERR_PENDING;
 
 	if (conn_xprt_read0_pending(h2c->conn))
-		flags |= (CS_FL_REOS|CS_FL_READ_NULL);
+		flags |= CS_FL_REOS;
 
 	/* Wake all streams with ID > last */
 	node = eb32_lookup_ge(&h2c->streams_by_id, last + 1);