GitLab Duo setup script should configure AI Gateway Url

Script doesn't currently set AI Gateway URL: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/duo/developments/setup.rb

The AI Gateway rake task sets export AI_GATEWAY_URL=http://0.0.0.0:5052 but that is deprecated now that we save the URL in the database: https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/lib/tasks/setup_ai_gateway.rake

Instead, we should remove references to AI_GATEWAY_URL and have Ai::Setting.current.ai_gateway_url be configured by the gitlab duo setup task.

This could also mean that we could remove our documentation about setting the AI Gateway URL in https://docs.gitlab.com/development/ai_features/