GPU: Cleanup obsolete settings and update deprecated CMake variable to new name

[davidrohr]

No description provided.

[github-actions]

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[alibuild]

Error while checking build/O2/fullCI_slc9 for a153a7a at 2025-06-16 00:02:

## sw/BUILD/O2Physics-latest/log
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:
Error in cling::AutoLoadingVisitor::InsertIntoAutoLoadingState:

Full log here.