Remove deleted packages from providers.json

Ideally this would be resolved with #47 (support renaming packages), but as a baseline Packagist should keep its `providers-*.json` current. If a package is delete through the Packagist UI, it should also disappear from there.

I can't demonstrate this issue on a specific package, since @Seldaek helpfully fixed the `silverstripe/` packages manually upon pinging him through IRC (thanks!). My previous example was `silverstripe/googlemapsbasic`

So if this has been fixed since, feel free to close this bug.