chore: nuke chat/completions API (#10157)

codex-rs/codex-api/src/endpoint/responses.rs

@@ -111,7 +111,6 @@ impl<T: HttpTransport, A: AuthProvider> ResponsesClient<T, A> {
     fn path(&self) -> &'static str {
         match self.streaming.provider().wire {
             WireApi::Responses | WireApi::Compact => "responses",
-            WireApi::Chat => "chat/completions",
         }
     }