Remove unreachable return from blockHelperMissing helper

Closure compiler found an [unreachable return](https://github.com/emberjs/ember.js/blob/0e7cf03cca93c4d39bb2c6d7efef9043ab0c892c/packages/ember-handlebars/lib/ext.js#L218) in the `'blockHelperMissing'` helper.

This PR removes it. That said, I don't understand the code well enough to check if this is just a stray line or a bug, so someone please review.