br: confusing message of full backup

[BornChanger]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Run full backup with --ratelimit is specified.

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

Backup job is run in serial

3. What did you see instead (Required)

Confusing messages show in the output like

[2024/01/30 17:26:19.926 +08:00] [WARN] [logging.go:253] ["setting --ratelimitand--concurrencyat the same time, ignoring--concurrency: --ratelimit forces sequential (i.e. concurrency = 1) backup"] [ratelimit=134.2MB/s] [concurrency-specified=1]
[2024/01/30 17:26:25.222 +08:00] [INFO] [client.go:874] ["backup range started"] [range-sn=0] [startKey=74800000000000010B5F7201] [endKey=74800000000000010B5F72FB] [rateLimit=134217728] [concurrency=4]

4. What is your TiDB version? (Required)

[BornChanger]

/component br

[BornChanger]

/severity moderate

[BornChanger]

We need to remove the attribute of concurrency for the tikv pb request because it is never used.