statistics: gc the statistics correctly after drop the database (#57309)

[ti-chi-bot]

This is an automated cherry-pick of #57309

What problem does this PR solve?

Issue Number: close #57230

Problem Summary:

What changed and how does it work?

We overlooked handling the drop schema event in the stats DDL handler. In this PR, we'll address this by cleaning up the dropped tables one by one.

Check List

Tests

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

  • I checked and no code files have been changed.

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.

修复统计信息在整个数据库被删除后无法清理的问题
Fix the issue where statistics cannot be cleaned up after the entire database is deleted.

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[ti-chi-bot]

@ti-chi-bot: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
idc-jenkins-ci-tidb/build	dd66458	link	true	/test build
idc-jenkins-ci-tidb/check_dev	dd66458	link	true	/test check-dev
idc-jenkins-ci-tidb/unit-test	dd66458	link	true	/test unit-test

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[0xPoe]

I am going to close this PR. Because on v7.5 we have to rewrite the code at all. Because the drop schema is a very rare operation. So I think it is not worth it to rewrite it again, and this bug will not affect the user's logic or stats at all. So I think it is OK to close it.

[ti-chi-bot]

This cherry pick PR is for a release branch and has not yet been approved by triage owners.
Adding the do-not-merge/cherry-pick-not-approved label.

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1. It must be approved by the approvers firstly.
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