Remove GitHub import deprecated workers

What does this MR do and why?

ImportReleaseAttachmentsWorker app/workers/gitlab/github_import/import_release_attachments_worker.rb changed it's location to app/workers/gitlab/github_import/attachments/import_release_worker.rb in %15.5 (~7months ago) in the scope of MR

Current MR:

• Removes app/workers/gitlab/github_import/import_release_attachments_worker.rb that is not used anymore.
• Documentation Stage::ImportAttachmentsWorker is updated since it was referring wrong workers

Corresponding issue GitHub Import: move pull request workers and im... (#377059 - closed)

MR acceptance checklist

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