Consider removing polyfills

[nsivertsen]

Maybe, for the upcoming 2.0 release, one could consider removing the fetch and Promise polyfills from prismic-javascript. They account for almost half of the bundle size. Both fetch and Promise have pretty good browser support by now, and most apps already have some kind of polyfill strategy in place (e.g. polyfill.io), meaning users are forced to download polyfills twice in many cases.