remove conversation history widget (#1727)

this widget is no longer used.

codex-rs/tui/src/lib.rs

@@ -24,11 +24,9 @@ mod app;
 mod app_event;
 mod app_event_sender;
 mod bottom_pane;
-mod cell_widget;
 mod chatwidget;
 mod citation_regex;
 mod cli;
-mod conversation_history_widget;
 mod exec_command;
 mod file_search;
 mod get_git_diff;
@@ -37,7 +35,6 @@ mod history_cell;
 mod insert_history;
 mod log_layer;
 mod markdown;
-mod scroll_event_helper;
 mod slash_command;
 mod status_indicator_widget;
 mod text_block;