Remove the styleguide

The idea was that this would be a place to document all the design
patterns used in Notify. However it hasn’t been kept up to date, and,
looking at the `git blame`[1] no new patterns have been added for 5
years.

I think it’s better to get rid of it than have to keep maintaining
something which is inaccurate.

1. 64aa0d359c/app/templates/views/styleguide.html

app/main/__init__.py

@@ -35,7 +35,6 @@ from app.main.views import (  # noqa isort:skip
     service_settings,
     sign_in,
     sign_out,
-    styleguide,
     templates,
     two_factor,
     tour,