br: remove totalFlashbackRegions since task completion region already contains all regions (#40504)

[ti-chi-bot]

This is an automated cherry-pick of #40504

What problem does this PR solve?

Issue Number: close #40391 #41005

Problem Summary:

What is changed and how it works?

refactoring code to remove totalFlashbackRegions since task completion region already contains all regions

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)

1. follow the doc https://docs.pingcap.com/zh/tidb-in-kubernetes/stable/backup-to-aws-s3-by-snapshot to do the backup
2. follow the doc https://docs.pingcap.com/zh/tidb-in-kubernetes/stable/restore-from-aws-s3-by-snapshot to do the restore
3. restore success

• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Leavrth
• joccau

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[fengou1]

/test unit-test

[joccau]

LGTM

[joccau]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 63fca7c