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/tui/src/app_event.rs

@@ -489,22 +489,6 @@ pub(crate) enum AppEvent {
         text: String,
     },
 
-    /// Voice transcription finished for the given placeholder id.
-    #[cfg(not(target_os = "linux"))]
-    #[cfg_attr(not(feature = "voice-input"), allow(dead_code))]
-    TranscriptionComplete {
-        id: String,
-        text: String,
-    },
-
-    /// Voice transcription failed; remove the placeholder identified by `id`.
-    #[cfg(not(target_os = "linux"))]
-    TranscriptionFailed {
-        id: String,
-        #[allow(dead_code)]
-        error: String,
-    },
-
     /// Open the branch picker option from the review popup.
     OpenReviewBranchPicker(PathBuf),