Remove related code for ddl 'set tiflash mode'

[hongyunyan]

What problem does this PR solve?

Issue Number: ref #1175 pingcap/tiflash#5252

What is changed and how it works?

Because we remove the ddl 'set tiflash mode', thus we need to remove the related code about this ddl.

Check List

Tests

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

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change
• Has persistent data change

Side effects

• Possible performance regression
• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch
• Need to update the documentation
• Need to update the tidb-ansible repository
• Need to be included in the release note

Release note

• No release note

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• hi-rustin
• lichunzhu

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[hongyunyan]

/run-all-tests

[ti-chi-bot]

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[hongyunyan]

/cc @lichunzhu @hi-rustin

[lichunzhu]

LGTM

[Rustin170506]

/merge

[ti-chi-bot]

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

Commit hash: ea77538

[ti-chi-bot]

@hongyunyan: Your PR was out of date, I have automatically updated it for you.

At the same time I will also trigger all tests for you:

/run-all-tests

If the CI test fails, you just re-trigger the test that failed and the bot will merge the PR for you after the CI passes.

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