Add feature spec for delete note action

What does this MR do and why?

Closes Follow-up from "WI note action - assign/unassig... (#404422 - closed)

Screenshots or screen recordings

No UI changes

How to set up and validate locally

Run the test case bin/rspec spec/requests/api/graphql/work_item_spec.rb

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 #404422 (closed)