br: remove totalFlashbackRegions since task completion region already contains all regions

[fengou1]

What problem does this PR solve?

cherry pick pr 40504
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

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[fengou1]

/test unit-test

[joccau]

LGTM

[fengou1]

/merge

[ti-chi-bot]

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

Commit hash: f18eb87