Remove backend code for attention request notifications

What does this MR do and why?

It removes notification emails, system notes, dashboard count and todos. Also, removes services that are no longer needed.

Related to #366442 (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 #366442 (closed)