chore(typings): remove StringMap

This was a poorly typed attempt to mimic TypeScript's index signatures,
which we can use instead.
This eliminates a very strange type that we were exposing to users, but
not re-exporting through our public API.

Fixes #4483

modules/angular2/test/router/route_recognizer_spec.ts

@@ -10,7 +10,7 @@ import {
   SpyObject
 } from 'angular2/test_lib';
 
-import {Map, StringMap, StringMapWrapper} from 'angular2/src/core/facade/collection';
+import {Map, StringMapWrapper} from 'angular2/src/core/facade/collection';
 
 import {RouteRecognizer} from 'angular2/src/router/route_recognizer';
 import {ComponentInstruction} from 'angular2/src/router/instruction';