[codex] Remove unused Rust helpers (#17146)

## Summary

Removes high-confidence unused Rust helper functions and exports across
`codex-tui`, `codex-shell-command`, and utility crates.

The cleanup includes dead TUI helper methods, unused
path/string/elapsed/fuzzy-match utilities, an unused Windows PowerShell
lookup helper, and the unused terminal palette version counter. This
keeps the remaining public surface smaller without changing behavior.

## Validation

- `just fmt`
- `cargo test -p codex-tui -p codex-shell-command -p codex-utils-elapsed
-p codex-utils-fuzzy-match -p codex-utils-string -p codex-utils-path`
- `just fix -p codex-tui -p codex-shell-command -p codex-utils-elapsed
-p codex-utils-fuzzy-match -p codex-utils-string -p codex-utils-path`
- `git diff --check`

codex-rs/utils/elapsed/src/lib.rs

@@ -1,11 +1,4 @@
 use std::time::Duration;
-use std::time::Instant;
-
-/// Returns a string representing the elapsed time since `start_time` like
-/// "1m 15s" or "1.50s".
-pub fn format_elapsed(start_time: Instant) -> String {
-    format_duration(start_time.elapsed())
-}
 
 /// Convert a [`std::time::Duration`] into a human-readable, compact string.
 ///