Remove the zebrad download command

[teor2345]

Motivation

This is a cleanup after #7613, but it involves removing a zebrad command-line command.

Remove the zebrad download command, and everything that depends on it.
This includes zcash-params/Dockerfile, and anywhere it is used in workflows and the main Dockerfile.

Complex Code or Requirements

This is a breaking change for anyone that manually runs the zebrad download command. But we can't predict how it's used, and it doesn't create files any more, so it's best to just remove it, rather than silently making it not work.

Testing

CI should pass with the Dockerfiles and commands removed.