*: Remove variable tidb_enable_alter_placement

[lcwangchao]

What problem does this PR solve?

Issue Number: close #31053

Problem Summary:

Placement rule in SQL is going to be GA. Remove the system variable tidb_enable_alter_placement that will never be used.

What is changed and how it works?

Check List

Tests

• Unit test
• 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

system variable `tidb_enable_alter_placement` is removed

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• morgo
• xhebox

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[lcwangchao]

/run-unit-test

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/40c4df1d004e8a49471ba3c2ecc20343357ed608

[lcwangchao]

/run-mysql-test

[lcwangchao]

/run-mysql-test

[lcwangchao]

/merge

[ti-chi-bot]

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

Commit hash: a895dbd

[ti-chi-bot]

@lcwangchao: Your PR was out of date, I have automatically updated it for you.

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/run-all-tests

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