refactor(types): remove dead code and minor

[xxchan]

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What's changed and what's your intention?

minor

• make private some internal traits (due to my mysophobia)
• add comments

Checklist

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[xxchan]

• feat: add map type to scalar/array/expr #17690
• refactor(types): remove dead code and minor #17691 👈
• main

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[wangrunji0408]

LGTM!

[xxchan]

💦