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

tests/app/main/views/test_styleguide.py

@@ -1,4 +0,0 @@
-def test_styleguide_can_render(app_):
-    response = app_.test_client().get('/_styleguide')
-
-    assert response.status_code == 200