Removed deprecation warning for those who've opt-in to I18N_COMPILE_WITHOUT_HANDLEBARS

I also fixed a typo in the warning message. Changing: `Ember.I18n...` to `Ember.ENV...` in the message.

Please let me know if you have any concerns with this change. I made it because we had opt-in and were confused about seeing the warning. I believe ember typically does not warn about feature deprecation if you're not using that particular feature.

I'd be happy to add rebase this commit and add a point release version if you'd like (2.1.1).
I had this here so that clients wouldn't get a deprecation warning *each time they compiled a template*. How about this?
```js
Ember.warn("...", Ember.ENV.I18N_COMPILE_WITHOUT_HANDLEBARS !== undefined)
```