Update the api docs

[lukasmartinelli]

https://github.com/mapbox/vtvalidate/blob/master/API.md is currently out of date 🙇

[GretaCB]

👍 thanks for noticing this @lukasmartinelli ! Looks like I accidentally copied over the API.md from node-cpp-skel. The API docs in vtvalidate's README.md are up to date. Since this is a relatively small API surface, going to rely on the README and just delete this API.md file.