radv: remove dead shared variables

LLVM does this anyway, but for ACO we need to do it in NIR.

Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
40 jobs for !2009 with backend in 23 minutes and 6 seconds (queued for 6 seconds)
merge request