Delete a bunch of packages that are now on GitHub.

pkg/http_throttle/README.md

-`http_throttle` is middleware for the [http package][] that throttles the number
-of concurrent requests that an HTTP client can make.
-
-```dart
-// This client allows 32 concurrent requests.
-final client = new ThrottleClient(32);
-
-Future<List<String>> readAllUrls(Iterable<Uri> urls) {
-  return Future.wait(urls.map((url) {
-    // You can safely call as many client methods as you want concurrently, and
-    // ThrottleClient will ensure that only 32 underlying HTTP requests will be
-    // open at once.
-    return client.read(url);
-  }));
-}
-```
-
-[http package]: pub.dartlang.org/packages/http