Remove ASSET_HOST env var

This env var is no longer in active use since the introduction of Static
proxy [1] and usage of it will soon have problems with the Content
Security Policy [2].

[1]: https://github.com/alphagov/govuk_app_config/blob/7f060692720df50a27f6845f052b04eae2246226/lib/govuk_app_config/govuk_proxy/static_proxy.rb
[2]: alphagov/govuk_app_config#274

config/application.rb

@@ -125,10 +125,6 @@ class Application < Rails::Application
     # https://github.com/alphagov/govuk-frontend/issues/1350
     config.assets.css_compressor = nil
 
-    # allow overriding the asset host with an enironment variable, useful for
-    # when router is proxying to this app but asset proxying isn't set up.
-    config.asset_host = ENV["ASSET_HOST"]
-
     # Do not swallow errors in after_commit/after_rollback callbacks.
     # config.active_record.raise_in_transactional_callbacks = true
   end