MINOR: checks: use conn_data_shutw_hard() instead of call via xprt

Now we only rely on the connection to do the job. This makes the code
cleaner as we don't have any references to the transport shutdown
outside of the connection anymore.
diff --git a/src/checks.c b/src/checks.c
index 8a9eab5..f61bc9c 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -1293,14 +1293,13 @@
 	 * To avoid sending RSTs all the time, we first try to drain pending
 	 * data.
 	 */
-	if (conn->xprt && conn->xprt->shutw)
-		conn->xprt->shutw(conn, 0);
+	__conn_data_stop_both(conn);
+	conn_data_shutw_hard(conn);
 
 	/* OK, let's not stay here forever */
 	if (check->result == CHK_RES_FAILED)
 		conn->flags |= CO_FL_ERROR;
 
-	__conn_data_stop_both(conn);
 	task_wakeup(t, TASK_WOKEN_IO);
 	return;