Remove old/unused `*_to_dpla` modules (fixes #7677)

* Removes old `*_to_dpla` modules superseded by newer Mapper classes.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 instead.
* Modifies `IAMapper().map_intermediate_provider()` to not throw
a `KeyError` if `metadata/collection` does not exist in IA metadata.