config: enforce-mpp config item is useless in v5.0.4

[LittleFall]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set config:

[performance]
enforce-mpp=true

start tidb 5.0.4+

2. What did you expect to see? (Required)

mysql> explain select a+1 from t;
+-------------------------+---------+-----------+---------------+--------------------------------+
| id                      | estRows | task      | access object | operator info                  |
+-------------------------+---------+-----------+---------------+--------------------------------+
| Projection_3            | 2.00    | root      |               | plus(test.t.a, 1)->Column#3    |
| └─TableReader_5         | 2.00    | root      |               | data:TableFullScan_4           |
|   └─TableFullScan_4     | 2.00    | cop[tikv] | table:t       | keep order:false, stats:pseudo |
+-------------------------+---------+-----------+---------------+--------------------------------+
3 rows in set, 1 warning (0.00 sec)

mysql> show warnings;
+---------+------+-----------------------------------------------------------------------------+
| Level   | Code | Message                                                                     |
+---------+------+-----------------------------------------------------------------------------+
| Warning | 1105 | MPP mode may be blocked because there aren't tiflash replicas of table `t`. |
+---------+------+-----------------------------------------------------------------------------+
1 row in set (0.00 sec)

3. What did you see instead (Required)

no warning.

4. What is your TiDB version? (Required)

v5.0.4

[LittleFall]

it only effects v5.0.4.

v5.0.3 doesn't have enforce-mpp.

v5.1+ has a different config/variable framework, and this config is available.

[dveeden]

/retitle config: enforce-mpp config item is useless in v5.0.4

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.