variable: Remove WARN for variable tidb_enable_noop_variable (#36890)

[ti-srebot]

cherry-pick #36890 to release-6.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36918

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/36918:release-6.2-6be08109fb84

---

What problem does this PR solve?

Issue Number: close #36647

Problem Summary:

What is changed and how it works?

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

When set `tidb_enable_noop_variable` to `WARN`, an error will be returned

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/8ee63bc9020ef82a92c490df3c73e934efc289f5