store/tikv: remove unused function and private the variable shuttingDown

[AndreMouche]

Signed-off-by: AndreMouche

What problem does this PR solve?

This PR removes unused function in tikv and private the variable ShuttingDown since the set/get function already exists.

Part of #22513

Check List

Tests

• Unit test

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• disksing
• youjiali1995

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[disksing]

/merge

[ti-chi-bot]

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

Commit hash: 54507dd

[ti-chi-bot]

@AndreMouche: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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[Rustin170506]

/run-check_dev_2