Remove model layer pieces from tasks to be done concept

What does this MR do and why?

Remove model layer pieces from tasks to be done concept

• no longer used in the codebase, so we'll remove it.
• table and final worker removal to be done in a subsequent MR.
• remove no longer valid rubocop todos

See #424804 (closed)

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 #424804 (closed)