AnalyzePartitionMergeConcurrency cannot be abled

[hawkingrei]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set tidb_merge_partition_stats_concurrency=10;

the code of tidb_merge_partition_stats_concurrency never is covered.

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

we will create 10 worker to do it.

3. What did you see instead (Required)

we will create 1 worker to do it.

4. What is your TiDB version? (Required)