remove system variable tidb_track_aggregate_memory_usage (pingcap#5029

)

system-variables.md

@@ -1084,16 +1084,6 @@ SET tidb_slow_log_threshold = 200;
 - Default value: 0
 - This variable is used to limit the maximum number of requests TiDB can send to TiKV at the same time. 0 means no limit.
 
-### tidb_track_aggregate_memory_usage <span class="version-mark">New in v5.0.0-rc</span>
-
-> **Warning:**
->
-> `tidb_track_aggregate_memory_usage` is currently an experimental feature. It is not recommended to use this feature in the production environment.
-
-- Scope: SESSION | GLOBAL
-- Default value: OFF
-- This variable controls whether to track the memory usage of the aggregate function. When you enable this feature, TiDB counts the memory usage of the aggregate function, which might cause the overall SQL memory statistics to exceed the threshold [`mem-quota-query`](/tidb-configuration-file.md#mem-quota-query), and then be affected by the behavior defined by [`oom-action`](/tidb-configuration-file.md#oom-action).
-
 ### tidb_txn_mode
 
 - Scope: SESSION | GLOBAL