ci/piglit-traces: drop re-definition of VK_DRIVER_FILES

It's already set higher up in that same file.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29201>

.gitlab-ci/piglit/piglit-traces.sh

@@ -118,8 +118,7 @@ else
       mkdir -p /tmp/.X11-unix
 
       env \
-        VK_DRIVER_FILES="/install/share/vulkan/icd.d/${VK_DRIVER}_icd.$(uname -m).json" \
-	weston -Bheadless-backend.so --use-gl -Swayland-0 --xwayland --idle-time=0 &
+        weston -Bheadless-backend.so --use-gl -Swayland-0 --xwayland --idle-time=0 &
 
       while [ ! -S "$WESTON_X11_SOCK" ]; do sleep 1; done
     }