Explicitly disable the unused ActionCable Rails feature

Just for consistency with the other features we're not using, we turn
off ActionCable when loading the app. I just removed
`config/cable.yml`, so I figure, let's not load a feature without the
config file it expects! (even though that didn't seem to bother it)

config/application.rb

@@ -9,11 +9,11 @@ require "rails"
 # Disabled:
 # - active_storage/engine
 # - active_job/railtie
+# - action_cable/engine
 # - action_mailbox/engine
 # - action_text/engine
 %w(
   active_record/railtie
-  action_cable/engine
   action_controller/railtie
   action_view/railtie
   action_mailer/railtie