remove dependency on discovery.yml

Closes: #229 (closed)

This removes the dependency on the discovery.yml. The current API is missing variants for the few (airflow, dbt, etc.) that were adding as part of https://gitlab.com/meltano/hub/-/merge_requests?scope=all&state=merged because we still overwrite the git backed files with the current state of the discovery.yml during the build/deploy step.

Variants added in the hub but not in discovery.yml !226 (comment 898117809).

Also see !226 (comment 901786382) for a detailed explaination.