util: remove ast package

[djshow832]

What problem does this PR solve?

Issue Number: close None

Problem Summary:
pkg/util/ast/ast_util.go is no more used now.

What is changed and how it works:
Remove pkg/util/ast/ast_util.go.

Check List

Tests

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

Notable changes

• Has configuration change
• Has HTTP API interfaces change (Don't forget to add the declarative for API)
• Has weirctl change
• Other user behavior changes

Release note

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None