Remove `ci_enable_scheduled_build` feature flag
After we made sure the feature works properly in https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/5223, we're going to remove the feature flag ci_enable_scheduled_build.
Ideally, this should be done by 22nd. => Since the evaluation term won't be done until end of October, we should include this %11.5 milestone, which implies to remove the FF by Nov. 8th.
Edited by Shinya Maeda