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/Cargo.toml

@@ -50,7 +50,6 @@ image = { workspace = true, features = ["jpeg", "png"] }
 itertools = { workspace = true }
 lazy_static = { workspace = true }
 mcp-types = { workspace = true }
-path-clean = { workspace = true }
 pathdiff = { workspace = true }
 pulldown-cmark = { workspace = true }
 rand = { workspace = true }