remove old, pre-reconfigurator upgrade APIs

[iliana]

This removes:

• the Sled Agent API update_artifact, which was intended for Nexus to command Sled Agent to apply an arbitrary update, immediately, whatever that meant for the individual artifact.
• the Nexus internal API cpapi_artifact_download, which was intended for Sled Agent to be able to download an artifact from a local cache managed by Nexus and expected to be long-lived.
• the test which tested update_artifact.

The replacements for this functionality will be:

• the TUF Repo Depot, in which Nexus immediately pushes artifacts onto sleds so that it doesn't have to maintain persistent storage.
• Reconfigurator, which will plan and execute updates in a coordinated fashion across the rack.
• New/existing endpoints specific to artifact kinds for applying updates; for example for zones, Nexus will put a new zone configuration to Sled Agent that tells it to use a specific zone image from the Repo Depot.

I spotted that this cleanup is needed while working on #4411.