*: remove deprecated heartbeat code (#2121)

[ti-chi-bot]

This is an automated cherry-pick of #2121

What problem does this PR solve?

as title

What is changed and how it works?

Since "heartbeat" has been deprecated for a long time in DM 2.0 and we already have another way of calculating syncer's lag, I am trying to remove the unused code from the DM codebase.

Check List

Tests

• Unit test
• Integration test

Code changes

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

Side effects

• Increased code complexity
• Breaking backward compatibility

Related changes

• Need to cherry-pick to the release branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• Ehco1996
• glorv

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[lance6716]

/run-all-tests

[Ehco1996]

/merge

[ti-chi-bot]

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

Commit hash: 48343dc