planner: simplify plan cache code

[qw4990]

What problem does this PR solve?

Issue Number: ref #51407

Problem Summary: planner: simplify plan cache code

What changed and how does it work?

NO LOGICAL CHANGE, JUST REFACTOR

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.

None

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[tiprow]

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