chore: rm remote models fflag (#11699)

rm `remote_models` feature flag.

We see issues like #11527 when a user has `remote_models` disabled, as
we always use the default fallback `ModelInfo`. This causes issues with
model performance.

Builds on #11690, which helps by warning the user when they are using
the default fallback. This PR will make that happen much less frequently
as an accidental consequence of disabling `remote_models`.

codex-rs/app-server/tests/suite/v2/turn_start.rs

@@ -1871,7 +1871,7 @@ fn create_config_toml_with_sandbox(
     feature_flags: &BTreeMap<Feature, bool>,
     sandbox_mode: &str,
 ) -> std::io::Result<()> {
-    let mut features = BTreeMap::from([(Feature::RemoteModels, false)]);
+    let mut features = BTreeMap::new();
     for (feature, enabled) in feature_flags {
         features.insert(*feature, *enabled);
     }