ac/nir/cull: Alway remove zero-area triangles in face culling.

ac/nir/cull: Alway remove zero-area triangles in face culling.

The face culling algorithm should have been disabled for
conservative overestimation because it already
(mistakenly) removed some close-to-zero area triangles.

Now that the driver disables it in that case,
let's always remove zero-area triangles.
This only costs +2 SALU instructions.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20987>