Small changes to buttons component - proposal

I've looked to the _buttons.scss components and I found a few things:

1. If we move some rules around between mixins and classes we can save some repetition.

2. I've checked against the code base and few button atoms (button--primary, --xsmall etc) we actually don't use at all. So I would like to remove it. I am happy to keep the documentation around it (so in the future devs will now how to create new modifications following the pattern) or move it to the module library but it's not necessary to send this to the user.

What do you think?