Use the default PostCSS browserlist config for autoprefixing

@kennethormandy fixed a problem with our PostCSS webpack config where because of how it was setup, you couldn't override the browserlist using a custom .browserslist file in your site. And by removing our custom option, we can now rely on the PostCSS default browserlist which is preferable as Gatsby doesn't have unique needs here so better to rely on the community maintaining a standard than us. #787