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.lock

@@ -1446,7 +1446,6 @@ dependencies = [
  "libc",
  "mcp-types",
  "opentelemetry-appender-tracing",
- "path-clean",
  "pathdiff",
  "pretty_assertions",
  "pulldown-cmark",
@@ -4277,12 +4276,6 @@ dependencies = [
  "path-dedot",
 ]
 
-[[package]]
-name = "path-clean"
-version = "1.0.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "17359afc20d7ab31fdb42bb844c8b3bb1dabd7dcf7e68428492da7f16966fcef"
-
 [[package]]
 name = "path-dedot"
 version = "3.1.1"