Remove styled-components dependency

[juandc]

Yes, I like it a lot, but probably not everybody wants to use styled-components for their styleguides.

Instead, styled-jsx is default supported by Next.js and would be used just for layout styling. No extra-broken dependencies 👍.

[jxnblk]

Thanks! I'd love to make this a little more flexible in regards to CSS, but trying to strike a balance between out-of-the-box smart defaults – that said, I'm not sure I'd want to use styled-jsx in this library