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

docs/typescript-definition-package/templates/angular2/angular2.d.ts.template.html

@@ -2,7 +2,6 @@
 {% block staticDeclarations %}
 
 interface Map<K,V> {}
-interface StringMap<K,V> extends Map<K,V> {}
 
 {% for alias, module in doc.moduleDocs %}
 declare module {$ module.namespace $} {