Stop compiling startup css in the asset pipeline

Since https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124406 they
are unused.

app/assets/stylesheets/application.scss

@@ -31,7 +31,3 @@
 @media print {
   @import 'print';
 }
-
-/* Rules for overriding cloaking in startup-general.scss */
-@import 'startup/cloaking';
-@include cloak-startup-scss(block);

config/application.rb

@@ -270,8 +270,6 @@ class Application < Rails::Application
     config.assets.precompile << "application_utilities_dark.css"
     config.assets.precompile << "application_dark.css"
 
-    config.assets.precompile << "startup/*.css"
-
     config.assets.precompile << "print.css"
     config.assets.precompile << "mailer.css"
     config.assets.precompile << "mailer_client_specific.css"