remove json_unquote() from expressions-pushed-down (#6211)

[ti-srebot]

cherry-pick #6211 to release-4.0
You can switch your code base to this Pull Request by using git-extras:

# In docs-cn repo:
git pr https://github.com/pingcap/docs-cn/pull/6214

After apply modifications, you can push your change to this PR via:

git push [email protected]:ti-srebot/docs-cn.git pr/6214:release-4.0-822fdc1ff71c

---

What is changed, added or deleted? (Required)

removed JSON_UNQUOTE() because this expression is not currently pushed down.

Which TiDB version(s) do your changes apply to? (Required)

• master (the latest development version)
• v5.0 (TiDB 5.0 versions)
• v4.0 (TiDB 4.0 versions)
• v3.1 (TiDB 3.1 versions)
• v3.0 (TiDB 3.0 versions)
• v2.1 (TiDB 2.1 versions)

What is the related PR or file link(s)?

• This PR is translated from:remove json_unquote() from expressions-pushed-down.md docs#5564
• Other reference link(s):

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

[ti-srebot]

@Liuxiaozhen12 please accept the invitation then you can push to the cherry-pick pull requests.
https://github.com/ti-srebot/docs-cn/invitations

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• TomShawn

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[TomShawn]

/merge

[ti-chi-bot]

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

Commit hash: 4521859