Remove ProjectExportWorker following global enablement of parallel project export

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Summary

Now that parallel_project_export feature has been rolled-out and the feature flag removed, we need to remove ProjectExportWorker.

While it is still used in the codebase, the ProjectTemplateExportWorker extends ProjectExportWorker so we can remove the ProjectExportWorker by making ProjectExportWorker the base class.

However, this will require refactoring a couple of specs which are still using ProjectExportWorker