do not set Action Cable's config when specify `--skip-action-cable` option

Also remove duplicated `config.action_cable.url` config.

railties/lib/rails/generators/rails/app/templates/config/environments/production.rb.tt

@@ -44,6 +44,9 @@ Rails.application.configure do
   # Action Cable endpoint configuration
   # config.action_cable.url = 'wss://example.com/cable'
   # config.action_cable.allowed_request_origins = [ 'http://example.com', /http:\/\/example.*/ ]
+
+  # Don't mount Action Cable in the main server process.
+  # config.action_cable.mount_path = nil
   <%- end -%>
 
   # Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
@@ -91,9 +94,5 @@ Rails.application.configure do
 
   # Do not dump schema after migrations.
   config.active_record.dump_schema_after_migration = false
-
-  # Don't mount Action Cable in the main server process.
-  # config.action_cable.mount_path = nil
-  # config.action_cable.url = "ws://example.com"
   <%- end -%>
 end