tiflash: Remove the description about expr_blacklist in use_tiflash.md (

#6188) (#6276)

tiflash/use-tiflash.md

@@ -232,12 +232,9 @@ set @@session.tidb_opt_broadcast_join=1
 Currently, TiFlash supports pushing down a limited number of expressions, including:
 
 ```
-+, -, /, *, >=, <=, =, !=, <, >, ifnull, isnull, bitor, in, bitand, or, and, like, not, case when, month, substr, timestampdiff, date_format, from_unixtime, json_length, if, bitneg, bitxor, 
-round without fraction, cast(int as decimal), date_add(datetime, int), date_add(datetime, string), min, max, sum, count, avg, approx_count_distinct
++, -, /, *, >=, <=, =, !=, <, >, ifnull, isnull, bitor, in, bitand, or, and, like, not, case when, month, substr, timestampdiff, date_format, from_unixtime, json_length, if, bitneg, bitxor, round without fraction, cast(int as decimal), min, max, sum, count, avg, approx_count_distinct
 ```
 
-Among them, the push-down of `cast` and `date_add` is not enabled by default. To enable it, refer to [Blocklist of Optimization Rules and Expression Pushdown](/blocklist-control-plan.md).
-
 TiFlash does not support push-down calculations in the following situations:
 
 - Expressions that contain the `Time` type cannot be pushed down.