tui: drop citation rendering (#4855)

We don't instruct the model to use citations, so it never emits them.
Further, ratatui [doesn't currently support rendering links into the
terminal with OSC 8](https://github.com/ratatui/ratatui/issues/1028), so
even if we did parse citations, we can't correctly render them.

So, remove all the code related to rendering them.

codex-rs/tui/src/lib.rs

@@ -36,7 +36,6 @@ mod app_event_sender;
 mod ascii_animation;
 mod bottom_pane;
 mod chatwidget;
-mod citation_regex;
 mod cli;
 mod clipboard_paste;
 mod color;