variable: small cleanups

[morgo]

What problem does this PR solve?

Problem Summary:

This is a small cleanup following the merge of #24914 - and then the removal of this from tidb-tests.

I have also changed the default "socket"; it gets overwritten during TiDB startup, but by changing it to empty string we can use it to auto-generate docs.

What is changed and how it works?

What's Changed:

Minor cleanup

Related changes

• None

Check List

Tests

• Manual test (add detailed scripts or steps below)

Side effects

• None

Release note

• No release note

[morgo]

/run-check_dev_2

[jackysp]

LGTM

[ti-chi-bot]

[REVIEW NOTIFICATION]

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• jackysp
• wjhuang2016

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[jackysp]

/merge

[morgo]

/merge cancel

[morgo]

It looks like there is still code in tidb-test that tries to enable the option that this PR removes. I'll take a look tomorrow.

[morgo]

/run-all-tests --tidb-test=pr/1216

[morgo]

/run-all-tests --tidb-test=pr/1216

[morgo]

/merge

[ti-chi-bot]

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

Commit hash: fdbdccd

[morgo]

/run-integration-common-test

[ti-chi-bot]

@morgo: Your PR was out of date, I have automatically updated it for you.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