radv: remove dead variables after splitting per member structs

Otherwise, nir_lower_clip_cull_distance_arrays might report
wrong number of output clips/culls because it relies on
shader output variables and some of them might be dead.

This fixes a rendering issue with Dolphin and Super Mario
Sunshine.

Fixes: b0c643d8 ("spirv: Use NIR per-member splitting")
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107610


CC: 18.2 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
(cherry picked from commit 24ee5323)