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*.