ChatGPT plan connections no longer need a feature flag

Connecting a personal ChatGPT plan used to depend on an organization feature flag that hosted deployments kept off. The flag is gone: connecting the provider is the opt-in, and each connection already belongs only to the user who signs in.
Open Models → Providers → Connect provider → ChatGPT → Sign in with ChatGPT, approve plan use in ChatGPT, then pick a discovered model in the chat model picker. The composer shows Using your ChatGPT plan. Other users, shared defaults and Playground cannot use your connection.
See ChatGPT plan and #4329, #4547.