chore: drop built-in MCPs (#22173)

Drop something that was never used

codex-rs/features/src/tests.rs

@@ -145,13 +145,6 @@ fn responses_websocket_response_processed_is_under_development() {
     );
 }
 
-#[test]
-fn builtin_mcp_is_under_development() {
-    assert_eq!(Feature::BuiltInMcp.stage(), Stage::UnderDevelopment);
-    assert_eq!(Feature::BuiltInMcp.default_enabled(), false);
-    assert_eq!(feature_for_key("builtin_mcp"), Some(Feature::BuiltInMcp));
-}
-
 #[test]
 fn terminal_resize_reflow_is_experimental_and_enabled_by_default() {
     assert_eq!(