Use existing updateWorkItem mutation for removing WI

What does this MR do and why?

Previously we had two different mutations in the codebase doing the exact same thing. This MR merges them together and removes the other.

Fixes Follow-up from "Ability to remove parent/child ... (#367820 - closed)

Screenshots or screen recordings

No visual change.

How to set up and validate locally

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Related to #367820 (closed)