Delete unused external-helpers package

This hasn't been used since we went to the rollup build and use decorator-transforms instead of transpiling most class features.

Author: ef4

packages/external-helpers/lib/external-helpers.js

@@ -168,7 +168,6 @@ function packages() {
 
       // these packages are special and don't get included here
       'loader/**',
-      'external-helpers/**',
       'ember-template-compiler/**',
       'internal-test-helpers/**',