Revert "refactor transcript view to handle HistoryCells" (#3614)

Reverts openai/codex#3538
It panics on forking first message. It also calculates the index in a
wrong way.

codex-rs/tui/src/lib.rs

@@ -32,6 +32,7 @@ mod app;
 mod app_backtrack;
 mod app_event;
 mod app_event_sender;
+mod backtrack_helpers;
 mod bottom_pane;
 mod chatwidget;
 mod citation_regex;