Delete DefaultEmbedService class

What does this MR do and why?

Delete DefaultEmbedService class

Delete specs for the class

Delete usages in other files and class

Screenshots or screen recordings

NA

How to set up and validate locally

1. Pipeline should be 🍏
2. DefaultEmbedService should not show up in dashboard finder.

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)