fix: remove dependency on expanduser crate (#667)

In putting up https://github.com/openai/codex/pull/665, I discovered
that the `expanduser` crate does not compile on Windows. Looking into
it, we do not seem to need it because we were only using it with a value
that was passed in via a command-line flag, so the shell expands `~` for
us before we see it, anyway. (I changed the type in `Cli` from `String`
to `PathBuf`, to boot.)

If we do need this sort of functionality in the future,
https://docs.rs/shellexpand/latest/shellexpand/fn.tilde.html seems
promising.

codex-rs/core/Cargo.toml

@@ -17,7 +17,6 @@ codex-apply-patch = { path = "../apply-patch" }
 dirs = "6"
 env-flags = "0.1.1"
 eventsource-stream = "0.2.3"
-expanduser = "1.2.2"
 fs-err = "3.1.0"
 futures = "0.3"
 mime_guess = "2.0"