Remove unused projects.json endpoints

On gitlab-com/gl-infra/production#735 (closed) we encountered an uncaught exception for /explore.json. We couldn't find where (if anywhere) the UI invokes this endpoint or other /explore/*.json endpoints, and they're not part of our documented API. The following endpoints also seem to fit that description:

• /admin/projects.json
• /dashboard/projects.json
• /dashboard/projects/starred.json

If they are indeed unused, we should remove them.

/cc @stanhu