build: remove ember-cli-babel-plugin-helpers from dependencies

Author: Nathanael Dekeister

package.json

@@ -37,7 +37,6 @@
     "broccoli-debug": "^0.6.5",
     "broccoli-persistent-filter": "^3.1.2",
     "broccoli-plugin": "^4.0.3",
-    "ember-cli-babel-plugin-helpers": "^1.1.1",
     "ember-cli-version-checker": "^5.1.2",
     "fs-tree-diff": "^2.0.1",
     "hash-for-dep": "^1.5.1",