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/plan_item.rs

@@ -215,10 +215,7 @@ async fn collect_turn_notifications(
 }
 
 fn create_config_toml(codex_home: &Path, server_uri: &str) -> std::io::Result<()> {
-    let features = BTreeMap::from([
-        (Feature::RemoteModels, false),
-        (Feature::CollaborationModes, true),
-    ]);
+    let features = BTreeMap::from([(Feature::CollaborationModes, true)]);
     let feature_entries = features
         .into_iter()
         .map(|(feature, enabled)| {