refactor($locale): use en-us as generic built-in locale

Previously there was a custom built en-us locale that was included with
angular.js. This made likely that it would get out of sync with the real
en-us locale that is generated from the closure library.

This change removes that custom one and uses the generated one instead.
This also has the benefit of preventing the unwanted caught error on trying
to load `ngLocale` during angular bootstrap.

Closes #12462
Closes #12444
Closes #12134
Closes #8174