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/common/Cargo.toml

@@ -7,11 +7,10 @@ edition = "2024"
 workspace = true
 
 [dependencies]
-chrono = { version = "0.4.40", optional = true }
 clap = { version = "4", features = ["derive", "wrap_help"], optional = true }
 codex-core = { path = "../core" }
 
 [features]
 # Separate feature so that `clap` is not a mandatory dependency.
 cli = ["clap"]
-elapsed = ["chrono"]
+elapsed = []