Use ESM, drop build step

The API uses Babel and a [build step](https://github.com/canada-ca/tracker/blob/master/api/Dockerfile#L19) to allow the use of the modern [ECMAScript Module](https://hacks.mozilla.org/2018/03/es-modules-a-cartoon-deep-dive/) syntax.
Full ESM support has recently been [added to node](https://nodejs.org/api/esm.html#esm_modules_ecmascript_modules), meaning Babel is no longer needed. **This gives us the opportunity simplify the build, and drop a bunch of dependencies.**