Removal of unnecessary abstractions

[alexdunnjpl]

💪 Motivation

As a developer, abstractions increase complexity (often significantly) and should therefore be motivated by an concrete payoff.

📖 Additional Details

For example, doi-service provides abstractions to allow arbitrary output endpoints, but support for more than a single endpoint is no longer a need and is not foreseen as a possible future need. Ergo, these abstractions are no longer as justified as they might have been in the past.

@alexdunnjpl to propose changes in comments. @tloubrieu-jpl to accept/reject proposed changes. @alexdunnjpl to nuke accepted targets from orbit.

⚖️ Acceptance Criteria

The resulting code still passes unit tests.

⚙️ Engineering Details

This work will need to be implemented in as granular a manner as possible, preferably while no other work is being done. All PRs affecting pds_doi_service.core should be merged before implementation.