Remove github_import_extended_events feature flag

What does this MR do and why?

This change removes the github_import_extended_events feature flag and all the no longer used code.

The feature flag was enabled by default on GitLab 16.9.

Workers that are no longer used were updated to not perform any tasks following the Sidekiq best-practices. A follow-up MR will delete the files.

Related to: #435089 (closed)

Context

For context, on #433536 (closed), GitHub Import was updated to import the resources notes, merged_by, pull request reviews, and pull request review requests via events. The change is behind the github_import_extended_events feature flag. So, now that we are removing the feature flag, we can remove all code that imports such resources via the previous original approach.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

1. Trigger a GitHub Import
   1. Go to + Project
   2. Import Project
   3. Select GitHub Import
   4. Provide the access token
   5. Select the project to be imported
2. Check if no error occurs and resources were imported
   1. Check the import stats in the UI or via API by requesting GET /api/v4/projects/<PROJECT_ID>/import/
   2. Navigate the imported project and check if issues, merge requests, notes, and events were imported