[Specs] Remove all the obsolete specs.

sky/specs/utils.md

-Dart Utilities Used By dart:sky
-===============================
-
-The classes defined here are used internally by dart:sky but are
-pretty generic.
-
-```dart
-class Pair<A, B> {
-  const Pair(this.a, this.b);
-  final A a;
-  final B b;
-  int get hashCode => a.hashCode ^ b.hashCode;
-  bool operator==(other) => other is Pair<A, B> && a == other.a && b == other.b;
-}
-
-// MapOfWeakReferences can be implemented in C, using the C Dart API, apparently
-class MapOfWeakReferences<Key, Value> {
-  external operator[](Key key);
-  external operator[]=(Key key, Value value);
-  external bool containsKey(Key key);
-}
-```