Remove deleted packages from providers.json

[chillu]

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.