Remove static/a paths

This analytics path has been removed from static [1] and is therefore no
longer used. The configuration for it can now be removed.

I'm a bit baffled by the modules/govuk/templates/node/s_backend_lb/assets-carrenza.conf.erb
file and whether it's still relevant, but it seems a big tangent to
investigate this over something so small.

[1]: alphagov/static#2152

modules/govuk/templates/node/s_backend_lb/assets-carrenza.conf.erb

@@ -95,21 +95,13 @@ server {
   }
   <%- end -%>
 
-  <%- if scope.lookupvar('::aws_migration') %>
-  set $upstream_static <%= @upstream_ssl ? 'https' : 'http' %>://static.<%= @app_domain %>;
-  <%- end %>
-  location = /static/a {
-    <%- if scope.lookupvar('::aws_migration') %>
-    proxy_pass $upstream_static;
-    <%- else %>
-    proxy_pass <%= @upstream_ssl ? 'https' : 'http' %>://static.<%= @app_domain %>;
-    <%- end %>
-  }
-
   location /__canary__ {
     return 200;
   }
 
+  <%- if scope.lookupvar('::aws_migration') %>
+  set $upstream_static <%= @upstream_ssl ? 'https' : 'http' %>://static.<%= @app_domain %>;
+  <%- end %>
   location / {
     <%- if scope.lookupvar('::aws_migration') %>
     proxy_pass $upstream_static;

modules/govuk/templates/static_extra_nginx_config.conf.erb

@@ -1,11 +1,3 @@
-location = /static/a {
-  expires -1;
-  add_header Last-Modified "";
-
-  default_type text/plain;
-  return 200 '';
-}
-
 # 1stline use this URL in their zendesk template
 location = /static/gov.uk_logotype_crown.png {
   absolute_redirect off;

modules/router/templates/assets_origin.conf.erb

@@ -78,10 +78,6 @@ server {
   <%- end -%>
 
   set $upstream_static <%= @upstream_ssl ? 'https' : 'http' %>://static.<%= @app_domain %>;
-  location = /static/a {
-    proxy_pass $upstream_static;
-  }
-
   location /__canary__ {
     proxy_pass $upstream_static;
   }