planner/core: fix duplicate enum items (#26145)

[ti-srebot]

cherry-pick #26145 to release-5.0
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/26201

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

git push git@github.com:ti-srebot/tidb.git pr/26201:release-5.0-00e19642e55e

---

What problem does this PR solve?

Issue Number: close #25955

Problem Summary:
In this pr: #23203 , I support enum push down for only mpp exchanger. However, a month later this pr: #22686 is merged to support general enum type push-down. This leads to the elem lists are appended two times.

What is changed and how it works?

What's Changed:
Remove the #23203 code

Check List

Tests

• Unit test
• Integration test, See fullstack-test:add test to check if enum works tiflash#2384

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

• planner/core: fix duplicate enum items

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@hanfei1991 you're already a collaborator in bot's repo.

[hanfei1991]

This pr has no need to merge to release 5.0