Remove feature flag `ai_custom_model`

config/feature_flags/beta/ai_custom_model.yml

doc/administration/self_hosted_models/configuration_types.md

...ministration/self_hosted_models/configure_duo_features.md

doc/administration/self_hosted_models/index.md

...inistration/self_hosted_models/licensing_and_offerings.md

doc/administration/self_hosted_models/logging.md

doc/administration/self_hosted_models/related_topics.md

...ion/self_hosted_models/supported_llm_serving_platforms.md

...sted_models/supported_models_and_hardware_requirements.md

doc/administration/self_hosted_models/troubleshooting.md

doc/development/custom_models/index.md

doc/user/gitlab_duo/index.md

ee/app/controllers/admin/ai/self_hosted_models_controller.rb

...p/controllers/admin/ai/terms_and_conditions_controller.rb

ee/app/graphql/mutations/ai/feature_settings/base.rb

ee/app/graphql/mutations/ai/self_hosted_models/base.rb

...esolvers/ai/feature_settings/feature_settings_resolver.rb

...vers/ai/self_hosted_models/self_hosted_models_resolver.rb

ee/lib/ee/sidebars/admin/panel.rb

ee/lib/gitlab/ai/self_hosted/ai_gateway.rb