Repro steps:
1. Open an HDRP or a URP project
2. Go to Project Settings > Graphics > HDRP or URP Settings
3. Try to unassign the Global Settings asset by browsing and selecting 'None'
4. Observe the assed is still assigned

Expected outcome:
- The asset to be unassigned.

Notes:
- Since the asset is required, perhaps unassigning it could provide a warning message under it with a 'Fix It' button (akin to the HDRP Wizard settings) if the Global Settings asset belongs to the currently active render pipeline.
- There are use cases for unassigning this asset, like removing it for an inactive pipeline.