br: Remove statement from closure to prevent concurrent operation during backup system table

[joccau]

What problem does this PR solve?

Issue Number: close pingcap/tidb#29710

Cherry-pick from pingcap/tidb#29730

What is changed and how it works?

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has not been approved.

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.

[joccau]

/sig migrate

[joccau]

/component br

[ti-chi-bot]

@joccau: The label(s) component/br cannot be applied, because the repository doesn't have them.

In response to this:

/component br

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the ti-community-infra/tichi repository.

[joccau]

/run-integration-br-test

[kennytm]

@joccau why is this merged into the master branch, not release-5.1 or release-5.0?