message: remove next-sass from next.config.js
diff: const withPlugins = require('next-compose-plugins')
  -const sass = require('@zeit/next-sass')
  -const mdx = require('@zeit/next-mdx')({extension: /\.mdx?$/})
  +const mdx = require('@zeit/next-mdx')
  -module.exports = withPlugins([sass, mdx], {
  +module.exports = withPlugins([
  + mdx({extension: /\.mdx?$/})
  +], {
  /*
  * Note: Prefixing assets with the fully qualified deployment URL
  * makes them available even when the site is served from a path alias, as in
  