Since we're now forcing users to rely on `npm` directly for updates, we
can also get rid of plugin commands that attempt to
install/update/remove using npm programatically and require users to use
`npm` directly as well.
This commit removes the following commands:
- `plugins`
- `plugin install`
- `plugin update`
- `plugin remove`
Despite plugin related commands being removed, *the functionality that
scans for plugins and registers them remains intact*.
We added this because we thought that knowledge of the supported device types, along with the configuration procedures was going to be encoded in the CLI.
With device specs, this is not longer the case.
