Deprecate Purple & Orange buttons

[sophshep]

We are no longer using Purple as our color scheme for Business / Enterprise, and so we don't use the purple buttons in our codebase: https://github.com/github/github/search?q=btn-purple&unscoped_q=btn-purple They can probably be removed from Primer.

Similarly, we are not using orange buttons on Open Source pages anymore. These should definitely be removed from Primer since they aren't accessible.

[broccolini]

Also wondering if we can deprecate the Blue button too? I don't think we use that anywhere either.