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/Cargo.toml

@@ -142,7 +142,6 @@ os_info = "3.12.0"
 owo-colors = "4.2.0"
 paste = "1.0.15"
 path-absolutize = "3.1.1"
-path-clean = "1.0.1"
 pathdiff = "0.2"
 portable-pty = "0.9.0"
 predicates = "3"