[DevOverlay] Remove temporary header children (#74490)
### Why?
The header children were temporary because they will affect the test utils in the future, we need to change these query selectors from `data-nextjs-dialog-header` to `data-nextjs-dialog-body`:
https://github.com/vercel/next.js/blob/bca0bc921bd81f25e6d8f83d676774724a007bfa/test/lib/next-test-utils.ts#L983-L986
This is not a severe blocker, and we may follow up on this during the QA period when we enable CI tests for the new UI.
We may even have to unify the target attributes for testing before the change.
