ha: remove unused etcd task cfg key

[Ehco1996]

What problem does this PR solve?

as title

What is changed and how it works?

after this #1323, we don't store taskConfig in etcd, so we can remove that code

Check List

Tests

• No code

Code changes

Side effects

Related changes

• Need to cherry-pick to the release branch

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• lance6716
• lichunzhu

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[lichunzhu]

/merge

[ti-chi-bot]

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

Commit hash: 8bba9ab

[ti-chi-bot]

In response to a cherrypick label: new pull request created: #2193.

[GMHDBJD]

We may need to add a TODO: prune them in etcd for these unused keys, otherwise these keys will remain forever when the user upgrade from an old cluster.