Remove contexts from service land.

Contexts existed from a time before we were using mongoengine and made
things easier at the time. Now that we use objects everywhere, just pass
them into services. This eliminates the need for contexts entirely.

This means things like run_triage() and run_service() need to be changed
everywhere.

Move is_pe() and is_pdf() into the Sample class.

Remove LocalAnalysisSource and LocalAnalysisDestination classes, they were
never used and never fully implemented.

Remove _delete_all_analysis_results(), it was never used anywhere.