Remove orbit_user_preference feature flag

What does this MR do and why?

Removes the orbit_user_preference feature flag. It has been default_enabled: true (beta) since 19.3, so this keeps the current behaviour and drops the flag-off code paths.

Related to #598922 (closed)

Post-merge

• Run the chatops feature delete orbit_user_preference cleanup, then close the rollout issue.

How to set up and validate locally

• bin/rspec ee/spec/models/ai/orbit/settings_spec.rb ee/spec/controllers/ee/profiles/preferences_controller_spec.rb ee/spec/services/ai/duo_workflows/mcp_config_service_spec.rb
• yarn jest ee/spec/frontend/ai/duo_agentic_chat/components/orbit_toggle_spec.js

Agent context

• Deleted the flag definition and the push_frontend_feature_flag call.
• Removed Ai::Orbit::Settings.user_preference_flag_enabled? and the legacy "every workflow gets Orbit tools" early return in subsetting_enabled?. The preferences controller now permits Orbit params whenever they changed.
• Frontend: dropped glFeatures.orbitUserPreference from orbit_toggle.vue and duo_agentic_chat_state_manager_light.vue.
• Specs: deleted flag-off contexts and kept the flag-on expectations.
• Ran the touched rspec files (831 examples, 0 failures), the two jest specs, rubocop and eslint.

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.