refactor: Removing `no_code_automation` category and its related code

What does this MR do and why?

Depends on gitlab-com/runbooks!6613 (merged). Closes gitlab-com/gl-infra/scalability#2658 (closed).

The no_code_automation category does not exist anymore and has no feature implemented, only a "do nothing" controller, worker, and model. This code was behind a feature flag (no_code_automation_mvc) in staging--now disabled.

@johnhope asked for the patch file containing the removal, so anyone interested in continuing with the work could make use of it.

• remove no_code_automation related code--not being used at the moment
• disable no_code_automation_mvc feature flag in staging--not in use in production
• update changelog
• patch file of the removal--attached to epic for posterity
  • Backend patch
  • Frontend patch
• Follow up issue to drop automation_rules table

Changelog: removed

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