Remove TUI voice transcription feature (#16114)

Removes the partially-completed TUI composer voice transcription flow,
including its feature flag, app events, and hold-to-talk state machine.

codex-rs/features/src/lib.rs

@@ -172,8 +172,6 @@ pub enum Feature {
     Artifact,
     /// Enable Fast mode selection in the TUI and request layer.
     FastMode,
-    /// Enable voice transcription in the TUI composer.
-    VoiceTranscription,
     /// Enable experimental realtime voice conversation mode in the TUI.
     RealtimeConversation,
     /// Removed compatibility flag. The TUI now always uses the app-server implementation.
@@ -813,12 +811,6 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
-    FeatureSpec {
-        id: Feature::VoiceTranscription,
-        key: "voice_transcription",
-        stage: Stage::UnderDevelopment,
-        default_enabled: false,
-    },
     FeatureSpec {
         id: Feature::RealtimeConversation,
         key: "realtime_conversation",