Remove unused `remove_file` method

In #32780, We have supported the `--skip-sprockets` option in the `app:update` task.
When `options[:api]` is truthy, `option[:skip_sprockets]` is also truthy. So we can remove this `remove_file` method.
42b9e7e50c/railties/lib/rails/generators/rails/app/app_generator.rb (L281-L283)

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -159,10 +159,6 @@ module Rails
           remove_file "config/initializers/cookies_serializer.rb"
         end
 
-        unless assets_config_exist
-          remove_file "config/initializers/assets.rb"
-        end
-
         unless csp_config_exist
           remove_file "config/initializers/content_security_policy.rb"
         end