*: remove non dry sysvar code

[morgo]

What problem does this PR solve?

Issue Number: close #33896

Problem Summary:

When setting a special sysvar that requires the domain, there is duplicate code in executor/set.go and domain/sysvar_cache.go. This is because a SetGlobal func can not be directly used, because the domain is not available.

It is possible to workaround this by creating a noop func in variable which is overwritten when the NewDomain is created. This helps simplify the code because by being non-DRY it has a high chance of introducing future bugs if only one location is updated correctly. Because the sysvar cache code runs async, it is also unlikely that these bugs will easily be caught.

What is changed and how it works?

Function neutral change.

Check List

Tests

• Unit test
  Covered by existing tests.
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

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

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• CbcWestwolf
• xhebox

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[YangKeao]

LGTM

[ti-chi-bot]

@YangKeao: Thanks for your review. The bot only counts approvals from reviewers and higher roles in list, but you're still welcome to leave your comments.

In response to this:

LGTM

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[YangKeao]

Good job! I'll revert #38227 after this PR merged 🍻

[YangKeao]

@xhebox Could you help to merge this PR?

[xhebox]

@xhebox Could you help to merge this PR?

This PR needs configuration OWNER to approve and merge, /cc @bb7133

[bb7133]

LGTM

[bb7133]

/merge

[ti-chi-bot]

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

Commit hash: 0727a82

[morgo]

/run-build

[sre-bot]

TiDB MergeCI notify

🔴 Bad News! New failing [1] after this pr merged.
These new failed integration tests seem to be caused by the current PR, please try to fix these new failed integration tests, thanks!

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci-tidb/integration-ddl-test	🟥 failed 1, success 5, total 6	4 min 24 sec	New failing
idc-jenkins-ci-tidb/common-test	🔴 failed 1, success 10, total 11	8 min 45 sec	Existing failure
idc-jenkins-ci/integration-cdc-test	🟢 all 37 tests passed	26 min	Existing passed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	11 min	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	5 min 24 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 35 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	3 min 55 sec	Existing passed
idc-jenkins-ci-tidb/mybatis-test	🟢 all 1 tests passed	3 min 14 sec	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	2 min 45 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed