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/navigation.py

@@ -321,7 +321,6 @@ class HeaderNavigation(Navigation):
         'show_accounts_or_dashboard',
         'sign_out',
         'start_job',
-        'styleguide',
         'submit_request_to_go_live',
         'template_history',
         'template_usage',
@@ -666,7 +665,6 @@ class MainNavigation(Navigation):
         'sign_in',
         'sign_out',
         'start_job',
-        'styleguide',
         'support',
         'support_public',
         'suspend_service',
@@ -978,7 +976,6 @@ class CaseworkNavigation(Navigation):
         'sign_in',
         'sign_out',
         'start_job',
-        'styleguide',
         'submit_request_to_go_live',
         'support',
         'support_public',
@@ -1291,7 +1288,6 @@ class OrgNavigation(Navigation):
         'sign_in',
         'sign_out',
         'start_job',
-        'styleguide',
         'submit_request_to_go_live',
         'support',
         'support_public',