Remove extra babel transform when upstream fixes land

[davewasmer]

Babel's regenerator implementation has a bug around nested arrow functions. So we added the arrow function transform to Babel so that the regenerator wouldn't encounter nested arrow functions. Once this PR lands, that bug will be fixed in Babel, so we can remove the redundant arrow transform.

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We also added the class transform because async functions don't support super via babel: babel/babel#3930

[davewasmer]

The PR mentioned here has landed, so this is ready to be fixed

[davewasmer]

Closing these out since they belong in denali-babel.

[davewasmer]

Also, the class transform mentioned above has already been removed from denali-babel. The mentioned bug has not been fixed, but Babel's ES5 transpiled classes can't interop with native ES6 classes, so it's a shortcoming Babel users will have to deal with.