chore: remove the REPL crate/subcommand (#754)

@oai-ragona and I discussed it, and we feel the REPL crate has served
its purpose, so we're going to delete the code and future archaeologists
can find it in Git history.

codex-rs/cli/Cargo.toml

@@ -20,7 +20,6 @@ anyhow = "1"
 clap = { version = "4", features = ["derive"] }
 codex-core = { path = "../core" }
 codex-exec = { path = "../exec" }
-codex-repl = { path = "../repl" }
 codex-tui = { path = "../tui" }
 serde_json = "1"
 tokio = { version = "1", features = [