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/connection.h b/include/types/connection.h
index 66a0f46..c7d968c 100644
--- a/include/types/connection.h
+++ b/include/types/connection.h
@@ -87,8 +87,8 @@
 	CS_FL_ERR_PENDING   = 0x00000800,  /* An error is pending, but there's still data to be read */
 	CS_FL_EOS           = 0x00001000,  /* End of stream delivered to data layer */
 	CS_FL_REOS          = 0x00002000,  /* End of stream received (buffer not empty) */
-	CS_FL_READ_NULL     = 0x00004000,  /* Last read received */
-	CS_FL_EOI           = 0x00008000,  /* end-of-input reached */
+	CS_FL_EOI           = 0x00004000,  /* end-of-input reached */
+	/* unused: 0x00008000 */
 	CS_FL_WAIT_FOR_HS   = 0x00010000,  /* This stream is waiting for handhskae */
 	CS_FL_KILL_CONN     = 0x00020000,  /* must kill the connection when the CS closes */