Delete the PostSetupUser API

[danxmoran]

PostSetupUser seems to be a shortcut API to create a new org + new bucket + new user with owner permissions on the two. It's similar to PostSetup, except that API grants the new user operator permissions on the entire deployment. The API has been part of our public Swagger file, but the CLI command that uses it hasn't been documented.

@russorat and I aren't sure what the purpose of the API is supposed to be. I think it might have been created back when OSS and Cloud shared a codebase, to support the onboarding path in Cloud. The API has since been locked down in Cloud, and other APIs exist in OSS to grant the same behavior, so we'd like to delete the API from OSS.

• Delete PostSetupUser from our public API docs
• Delete all server-side handlers for the API

The influx setup user subcommand will not be migrated to the new CLI repo.