Don't load the admin's locales unless you explicity register the admin module and remove AR version of locales

Fixes #1701.

If you're using `Padrino::Admin::AccessControl`, the extension `Padrino::Admin` gets registered for you. Otherwise, if you want to use the locales you have to add `register Padrino::Admin` to your `app/app.rb`.

I've also removed our version of `ActiveRecord` locales.