TiFlash: remove outdated tune advise

tiflash/tune-tiflash-performance.md

@@ -52,14 +52,4 @@ If you want to save machine resources and have no requirement on isolation, you
 
     ```sql
     set @@tidb_opt_distinct_agg_push_down = 1;
-    ```
-
-5. If the `JOIN` operator does not choose the MPP mode, you can modify the value of `tidb_opt_network_factor` to make the`JOIN` operator choose the MPP mode:
-
-    The variable `tidb_opt_network_factor` is used to set the ratio of network overhead that the optimizer takes into account when calculating the cost. The smaller the variable value is, the smaller the estimated cost for a large amount of network transmissions is, and the more TiDB inclined to choose the MPP operator.
-
-    {{< copyable "sql" >}}
-
-    ```sql
-    set @@tidb_opt_network_factor = 0.001;
-    ```
+    ```