Stop transforming async to generators

[jasonLaster]

It'll be faster to use native async/await, so we should be able to drop transform-async-to-generator

[tromey]

I wonder if we can remove some of the other "es2015" transforms as well.

[clarkbw]

Here's a list of the plugins from launchpad

• [ ] "transform-es2015-modules-commonjs"
• [ ] "transform-es2015-block-scoping"
• [ ] "transform-es2015-destructuring"
• [ ] "transform-es2015-parameters"
• [ ] "transform-es2015-spread"
• [ ] "transform-flow-strip-types"
• [x] "transform-async-to-generator"