Remove free user cap email notification related workers

What does this MR do and why?

This MR is the first step to remove the free user cap notification related workers:

1. Implement no-op in the two limited capacity sidekiq workers, and remove the code that used by the worker: services, mailer.
2. Remove the two feature flags that control the worker
3. Ignore the database columns used by the worker

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #415486