[Refactor][apiClient] It needs document or refactor in `AfterResponse`

[likyh]

What and why to refactor

It's confuse.

https://github.com/apache/incubator-devlake/blob/686dd0df4fc0f45cebe919e08087735730aed67e/plugins/github/tasks/api_client.go#L48-L53

apiClient.SetAfterFunction is called here but actually, it's no use, because it be reassign in here:

https://github.com/apache/incubator-devlake/blob/686dd0df4fc0f45cebe919e08087735730aed67e/plugins/helper/api_collector.go#L117-L126

Describe the solution you'd like

delete default value in api_collector.

[github-actions]

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