Implement manifest remove in CLI

[alilleybrinker]

Similar to #195, we want the ability to remove an existing manifest from the CLI. This should likely be doable with both the manifest identifier (or a unique prefix of it) or by the identifier of the target (since OmniBOR by-design requires that target artifacts have unique manifests describing their inputs. If a manifest is detached (meaning we don't know the target it's associated with), then it has to be removed only by its own identifier.