i18n: Drop empty, discontinued translation en_NG

We no longer have this in Transifex.  Our `tools/tx-sync` script
doesn't automatically remove stale files, instead just leaving
them unchanged.  Remove it now by hand.

The file was virtually identical to the `messages_en.json` from
when it was last updated:

  $ git diff -U0 77a645f:static/translations/messages_en.json \
      HEAD:static/translations/messages_en_NG.json

The only translation was of "Private messages" to
"Private massages".

Author: gnprice

flow-typed/translations.js

@@ -60,10 +60,6 @@ declare module '../../static/translations/messages_en_GB.json' {
   declare export default {| [string]: string |};
 }
 
-declare module '../../static/translations/messages_en_NG.json' {
-  declare export default {| [string]: string |};
-}
-
 declare module '../../static/translations/messages_eo.json' {
   declare export default {| [string]: string |};
 }

src/i18n/messages.js

@@ -25,7 +25,6 @@ export default {
   el: require('../../static/translations/messages_el.json'),
   en: require('../../static/translations/messages_en.json'),
   'en-GB': require('../../static/translations/messages_en_GB.json'),
-  'en-NG': require('../../static/translations/messages_en_NG.json'),
   eo: require('../../static/translations/messages_eo.json'),
   es: require('../../static/translations/messages_es.json'),
   fa: require('../../static/translations/messages_fa.json'),