chore: update exec crate to use std::time instead of chrono (#952)

When I originally wrote `elapsed.rs`, I realized we were using both
`std::time` and `chrono` with no real benefit of having both. We should
try to keep the `exec` subcommand trim (as it also buildable as a
standalone executable), so this helps tighten things up.

codex-rs/Cargo.lock

@@ -503,7 +503,6 @@ dependencies = [
 name = "codex-common"
 version = "0.0.0"
 dependencies = [
- "chrono",
  "clap",
  "codex-core",
 ]