Remove unused helper function normalizeAttributes

[diamondo25]

I was not able to find any references to normalizeAttribute:

➜  frontend git:(master) ✗ grep 'normalizeAttributes' -R --exclude-dir tmp . | grep -v \.map
./dist/assets/vendor.js:      @method normalizeAttributes
./dist/assets/vendor.js:    normalizeAttributes: function normalizeAttributes(typeClass, hash) {
./node_modules/ember-data/addon/serializers/json.js:    @method normalizeAttributes
./node_modules/ember-data/addon/serializers/json.js:  normalizeAttributes(typeClass, hash) {
./node_modules/ember-data/CHANGELOG.md:* Moved several normalize helper methods to the JSONSerializer - Move `normalizeAttributes` to the `JSONSerializer` (mirrors `serializeAttributes`) - Move `normalizeRelationships` to the `JSONSerializer` - Move `normalizePayload` to the `JSONSerializer`

[bmac]

Thanks @diamondo25.