[BUG] Unused old method for REST API client

[IvoTod]

What happened:
@livovachkov was writing tests for variant queries and stumbled upon the post_query_variants_download method wdae/wdae/remote/rest_api_client.py. It attempts to get the variants through a download requests and then incorrectly tries to interpet the CSV stream response as a JSON list stream and errors. Upon further inspection, the method is currently unused and has been left in this unusable and useless state for a long time.

What you expected to happen:
The method should at the very least work or be removed if it is deemed unnecessary.