Investigate `register-babel` necessity

I accidentally discovered that I no longer needed to manually register Babel, given that `gulpfile.babel.js` seemed to register it for me, so I removed that from the code in b404c5facb0c.

Shortly after, I decided to update the [generator-babel-boilerplate](https://github.com/babel/generator-babel-boilerplate/) with this change, but @thejameskyle pointed out that it may not be intended to work that way (https://github.com/babel/generator-babel-boilerplate/pull/240#issuecomment-167838422).

This issue to get to the bottom of this!

Start with commit b404c5facb0c and inspect how gulpfile.babel.js handles Babel registration. Compare that behavior with the generator-babel-boilerplate pull request #240 and its linked comment to determine the intended setup. Done means reaching a clear conclusion about whether manual registration is required and recording or implementing the agreed configuration change.

Written by the indexing model from the issue text.