Clean metrics/dashboard/* workers and update specs

What does this MR do and why?

Aids in cleaning up files mentioned in #397141 (closed)

Worker classes are removed as per direction mentioned in https://docs.gitlab.com/ee/development/sidekiq/compatibility_across_updates.html#removing-worker-classes

Screenshots or screen recordings

NA

How to set up and validate locally

Stuff should just work without these files.

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 #397141 (closed)