Removing the datarel package entirely has proved to be difficult (DM-2928, DM-2948), so instead I'm simply going to remove non-ingest code (and dead ingest code) from the package, along with its dependencies on ap and testing_endToEnd. Other dependencies will be retained even if they aren't necessary for the code that will remain in datarel, to support lsstDoxygen's use of datarel as a top-level package for documentation generation.