Delete CustomDashboardService class and other related files

What does this MR do and why?

Delete CustomDashboardService class

Delete UpdateDashboardService class

Update usages of both classes

Delete/update related specs

Screenshots or screen recordings

NA

How to set up and validate locally

1. The deleted dashboard class should not be present in the finder.
2. Pipeline should be 🍏

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 #397139 (closed)