Handle project deletion by also deleting in Zoekt

Right now we do nothing in Zoekt when a project is deleted. This means that we leave orphaned bare repos and index data laying around in Zoekt.

We will need to add a new endpoint to the zoekt-dynamic-indexserver like the ones added in https://github.com/sourcegraph/zoekt/pull/496 at /delete/:repoid which deletes the local bare repo and index files for this repo.