[codex] remove responses command (#19640)

This removes the hidden `codex responses` CLI subcommand after
confirming no downstream callers rely on it, deleting the raw Responses
passthrough implementation, unregistering the subcommand, and dropping
the now-unused CLI dependencies on `codex-api` and
`codex-model-provider`.

codex-rs/Cargo.lock

@@ -2099,7 +2099,6 @@ dependencies = [
  "assert_matches",
  "clap",
  "clap_complete",
- "codex-api",
  "codex-app-server",
  "codex-app-server-protocol",
  "codex-app-server-test-client",
@@ -2116,7 +2115,6 @@ dependencies = [
  "codex-login",
  "codex-mcp",
  "codex-mcp-server",
- "codex-model-provider",
  "codex-models-manager",
  "codex-protocol",
  "codex-responses-api-proxy",