Remove `GET /ubkg-download-file-list`

[yuanzhou]

Due to the recent changes in UBKG Download UI App, a new JSON schema was introduced to file_descriptions.json. We no longer need to call ingest-api's GET /ubkg-download-file-list to retrieve the files list.

• Remove this endpoint from the ingest-api codebase
• Also remove from the Gateway auth api_endpoints.*.json

[yuanzhou]

There's a chance that we may still keep it if the static json can't handle file size and last modified date properly.