remove 'en' exclusion in i18n.py

Based on Discussion in IRC:

philippkueng > Hi guys, in the ckan/lib/i18n.py file you have 2 exclusions for 'en' in the method handle_request and set_lang (if statements). ...we're currently extending the translation files and are using placeholders hence we need to have a translation file for 'en' aswell. Might it be possible to remove those 2 ifs for future releases? just before i create a PR.
johnglover > philippkueng: this sounds reasonable to me, would you mind making a Github issue for this?