Remove import of non-existent JS

The intervention banner Javascript was removed in the publishing components gem as that was no longer needed.

app/assets/javascripts/application.js

@@ -4,7 +4,6 @@
 //= require govuk_publishing_components/components/error-summary
 //= require govuk_publishing_components/components/feedback
 //= require govuk_publishing_components/components/govspeak
-//= require govuk_publishing_components/components/intervention
 //= require govuk_publishing_components/components/metadata
 //= require govuk_publishing_components/components/print-link
 //= require govuk_publishing_components/components/radio