*: Remove incorrect global sysvar caching

[morgo]

What problem does this PR solve?

Issue Number: close #24368

Problem Summary:

What is changed and how it works?

What's Changed:

Previously running SHOW VARIABLES would cache global-only sysvars, which was an incorrect behavior. The values are now no longer cached.

Related changes

• This depends on domain, session: Add new sysvarcache to replace global values cache #24359 (merged) - which is a higher performance cache for sysvars.

Check List

Tests

• Integration test

Side effects

• None

Release note

• Previously the value of global-only system variables may have been cached in a session. TiDB now no longer caches these values in the session, which is a more-compatible behavior with MySQL.

[morgo]

/run-all-tests

[zhouqiang-cl]

/run-all-tests

[xhebox]

/lgtm

[wjhuang2016]

/lgtm

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• wjhuang2016
• xhebox

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[morgo]

/merge

[ti-chi-bot]

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

Commit hash: 2adab1a

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@morgo: Your PR was out of date, I have automatically updated it for you.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

/run-integration-common-test