Remove deprecated govuk-button--disabled

See alphagov/govuk-frontend#2271

The only place where this is used is the download button for the provisional
assessment by date report on the MI page. The button is (and was) only
appearing as disabled but could still be pressed as it is a link and not a
button.

lib/govuk_component/button_helpers.rb

@@ -85,7 +85,6 @@ def govuk_link_button_warning(name = nil, options = nil, tag_options = {}, &)
     def disable_if(options)
       return unless options[:disabled].present? && options[:disabled].to_s == 'true'
       options[:aria] = { disabled: 'true' }
-      prepend_classes('govuk-button--disabled', options)
     end
 
     def govuk_link_button_options(options)