chore(config) rm experimental_use_freeform_apply_patch (#22565)

## Summary
Get rid of the `experimental_use_freeform_apply_patch` config option,
since it is now encoded in model config. No deprecation message since it
has been experimental this entire time.

## Testing
- [x] Updated unit tests

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/features/src/tests.rs

@@ -393,7 +393,6 @@ fn from_sources_applies_base_profile_and_overrides() {
         },
         FeatureOverrides {
             web_search_request: Some(false),
-            ..Default::default()
         },
     );