Cleanup unused/out of date code (fixes #7645, #7677)

* Remove thumbnail-related code (fixes #7645)
* Removes old `*_to_dpla` modules superseded by newer Mapper classes
(fixes #7677).
* Removes associated tests if no longer relevant, or changes
existing tests to use the `dpla_mapper` endpoint instead (refs #7677).
* Modifies `IAMapper().map_intermediate_provider()` to not throw
a `KeyError` if `metadata/collection` does not exist in IA metadata
(refs #7677).
* Rename Hathi test to reflect purpose and additional cleanup
* Delete old Harvard Virtual Campus profile
* Point test_enrich.py non-run tests at proper endpoint