borg2: prune vs delete

[ThomasWaldmann]

They work slightly different:

Deletion:

• prune: creates Archive instance first, then calls .delete()
• delete: only uses the manifest, avoids creating an Archive instance

Logging:

• prune: uses powerful archive formatter for logging (which also potentially creates an Archive instance, but offers a lot of formatting placeholders)
• delete: only uses archive_info and a very simple formatter (avoiding creation of an Archive instance for better robustness)

Check:

• where does the timestamp prune uses for its deletion policy come from, does it need to create an Archive instance (or load the archive metadata from repo otherwise) for that anyway?
• shall prune work in the same way as delete (making it more robust against defective archives maybe?)