Remove support for env config

[colmsnowplow]

Originally, we developed this app with the requirement that a config-file-free configuration must be possible (due to some of the deployment modes we began with).

This is no longer a requirement, and facilitating it adds more to maintenance overhead than one would expect - we must maintain code for two separate hcl decoders, for example.

Since HCL configs can support env vars, there's no reason to continue supporting the old env var configuration model. We should rip that support out and use the opportunity to rationalise the config code as much as possible.