Remove Gitlab::GithubImport::RefreshImportJidWorker
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!142980 moved `Gitlab::GithubImport::RefreshImportJidWorker` to a more generic namespace now that it's used by Bitbucket importers: `Gitlab::Import::RefreshImportJidWorker`. However, the old worker was left in place and simply enqueued the renamed worker, effectively making it a no-op per our [documentation on removing workers](https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#removing-worker-classes).
Once the initial MR is merged to production and `Gitlab::Import::RefreshImportJidWorker` is successfully being enqueued, `Gitlab::GithubImport::RefreshImportJidWorker` and its specs can safely be removed.
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