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/include/types/stream_interface.h b/include/types/stream_interface.h
index d1b8135..61937e0 100644
--- a/include/types/stream_interface.h
+++ b/include/types/stream_interface.h
@@ -72,7 +72,7 @@
 	SI_FL_NOLINGER   = 0x00000080,  /* may close without lingering. One-shot. */
 	SI_FL_NOHALF     = 0x00000100,  /* no half close, close both sides at once */
 	SI_FL_SRC_ADDR   = 0x00001000,  /* get the source ip/port with getsockname */
-	SI_FL_READ_NULL  = 0x00000200,   /* Last read reported */
+	/* unused: 0x00000200 */
 	SI_FL_WANT_GET   = 0x00004000,  /* a stream-int would like to get some data from the buffer */
 	SI_FL_CLEAN_ABRT = 0x00008000,  /* SI_FL_ERR is used to report aborts, and not SHUTR */