Artifact packages are not removed on uninstall

[aschempp]

I had a package installed through a ZIP artifact. When the extension is removed, the ZIP artifact remains on the server. This means if the package is later installed through other means (in my case our extensions.terminal42.ch packagist), Composer prefers the local ZIP artifact over what remote servers provide.

We should probably delete artifacts from contao-manager/packages if the package is uninstalled.