planner/core: fix duplicate enum items

[hanfei1991]

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]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lzmhhh123
• rebelice

To complete the pull request process, please ask the reviewers in the list to review by filling /cc @reviewer in the comment.
After your PR has acquired the required number of LGTMs, you can assign this pull request to the committer in the list by filling /assign @committer in the comment to help you merge this pull request.

The full list of commands accepted by this bot can be found here.

Details

Reviewer can indicate their review by submitting an approval review.
Reviewer can cancel approval by submitting a request changes review.

[hanfei1991]

/merge

[ti-chi-bot]

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

Details

Commit hash: e9beed9

[hanfei1991]

/run_check_dev_2

[hanfei1991]

/merge

[ti-srebot]

cherry pick to release-5.0 in PR #26201

[ti-srebot]

cherry pick to release-5.1 in PR #26202