Remove hooks on uninstalling package

[heiglandreas]

@ramsey asked whether the hooks can be uninstalled when uninstalling The Cap'n

https://twitter.com/ramsey/status/1266526935368314887?s=19

[ramsey]

Thanks, @heiglandreas!

[heiglandreas]

Now, with composer2.0 out we can actually do that!

Implementation upcoming...