chore: drop undo from the docs (#8431)

codex-rs/tui2/tooltips.txt

@@ -1,6 +1,5 @@
 Use /compact when the conversation gets long to summarize history and free up context.
 Start a fresh idea with /new; the previous session stays in history.
-If a turn went sideways, /undo asks Codex to revert the last changes.
 Use /feedback to send logs to the maintainers when something looks off.
 Switch models or reasoning effort quickly with /model.
 You can run any shell command from Codex using `!` (e.g. `!ls`)