chore(core) Remove Feature::PowershellUtf8 (#15128)

## Summary
This feature has been enabled for powershell for a while now, let's get
rid of the logic

## Testing
- [x] Unit tests

codex-rs/features/src/lib.rs

@@ -132,8 +132,6 @@ pub enum Feature {
     ChildAgentsMd,
     /// Allow the model to request `detail: "original"` image outputs on supported models.
     ImageDetailOriginal,
-    /// Enforce UTF8 output in Powershell.
-    PowershellUtf8,
     /// Compress request bodies (zstd) when sending streaming requests to codex-backend.
     EnableRequestCompression,
     /// Enable collab tools.
@@ -689,18 +687,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Removed,
         default_enabled: false,
     },
-    FeatureSpec {
-        id: Feature::PowershellUtf8,
-        key: "powershell_utf8",
-        #[cfg(windows)]
-        stage: Stage::Stable,
-        #[cfg(windows)]
-        default_enabled: true,
-        #[cfg(not(windows))]
-        stage: Stage::UnderDevelopment,
-        #[cfg(not(windows))]
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::EnableRequestCompression,
         key: "enable_request_compression",