remove conversation history widget (#1727)

this widget is no longer used.

codex-rs/tui/src/app_event.rs

@@ -20,10 +20,6 @@ pub(crate) enum AppEvent {
     /// Text pasted from the terminal clipboard.
     Paste(String),
 
-    /// Scroll event with a value representing the "scroll delta" as the net
-    /// scroll up/down events within a short time window.
-    Scroll(i32),
-
     /// Request to exit the application gracefully.
     ExitRequest,