Remove support for GCP auth via env vars

[colmsnowplow]

Originally, we allowed the user to provide a b64 encoded representation of GCP service account credentials, we stored that in a file and set the GOOGLE_APPLICATION_CREDENTIALS env var to the path of that file. The reason for this was to support AWS Lambda and GCP cloudfunction deployments.

In release/1.0.0, we're ripping out support for those deployments: #140, because this app is largely not well designed for those kinds of deployments.

So, we can remove that piece of logic, and just expect the user to make the credentials file available to the app (eg by mounting to the container), and just set the GOOGLE_APPLICATION_CREDENTIALS env var themselves.

Discussion: #143 (comment)