Remove test for passwd removal

Configs with passwd are still accepted in 2.3.

maxscale-system-test/config_test.cpp

@@ -21,7 +21,8 @@ const char* bad_configs[] =
     "mxs720_wierd_line",
     "mxs799",
     "mxs1731_empty_param",
-    "old_passwd",
+// passwd is still supported
+//    "old_passwd",
     NULL
 };