Eliminate defaults in config.py

[theferrit32]

We integrate with dotenv already, and automatically load from .dev.env. For some fields we have a default specified both in config.py and in .dev.env, and for some we don't.

All env defaults specified in config.py should be moved out into .dev.env

Also e.g. CLINVAR_INGEST_RELEASE_TAG should not have a default value. This changes relatively frequently and the application should be required to specify its own.

https://github.com/clingen-data-model/clinvar-ingest/blob/main/clinvar_ingest/config.py