Remove legacy ModelInfo and merge it with ModelFamily (#7748)

This is a step towards removing the need to know `model` when
constructing config. We firstly don't need to know `model_info` and just
respect if the user has already set it. Next step, we don't need to know
`model` unless the user explicitly set it in `config.toml`

codex-rs/core/src/lib.rs

@@ -67,7 +67,6 @@ pub use conversation_manager::NewConversation;
 pub use auth::AuthManager;
 pub use auth::CodexAuth;
 pub mod default_client;
-mod openai_model_info;
 pub mod project_doc;
 mod rollout;
 pub(crate) mod safety;