Remove unused code (fixes #7645 and #7677)

* Remove thumbnail-fetching related code.
* Removes old `*_to_dpla` modules superseded by newer `Mapper` classes.
* Removes associated tests if no longer relevant, or changes existing tests to use the `dpla_mapper` endpoint or relevant `Mapper` classes instead.
* Modifies `IAMapper().map_intermediate_provider()` to not throw a `KeyError` if `metadata/collection` does not exist in IA metadata.
* Removes `profiles/virtual.pjs`, an old Harvard profile no longer in use.