When unlinking installation from organization, delete ProjectGithubRepos

# Problem

Unlinking a `GithubAppInstallation` from an `Organization` deletes the related `OrganizationGithubAppInstallation` record.

That, however, currently in no way affects `ProjectGithubRepo` records which may link a `GithubRepo` belonging to the `GithubAppInstallation` with a project belonging to the `Organization.`

We should also delete the related `ProjectGithubRepo` records when the installation is unlinked.

Trace the unlinking flow for GithubAppInstallation and OrganizationGithubAppInstallation, then inspect how ProjectGithubRepo connects GithubRepo records to projects. Ensure unlinking also removes affected ProjectGithubRepo records, and verify the behavior with the relevant unlinking or association tests; the issue is done when those records no longer remain after unlinking.

Written by the indexing model from the issue text.