Remove sast_configuration_ui feature flag

This issue tracks the removal of the sast_configuration_ui feature flag.

It was enabled-by-default in #231357 (closed).

Implementation plan

• Remove feature flag from https://gitlab.com/gitlab-org/gitlab/blob/v13.2.0-rc44-ee/ee/app/controllers/projects/security/sast_configuration_controller.rb#L34
• Remove feature flag from deployments:
  • Staging
  • [-] Canary This is the same as prod!
  • Production
• Clean up all conditional logic branches based on feature flag
  • Tests
  • Components