zink: do not leave needless shader temps around

zink: do not leave needless shader temps around

This used to not matter, but since we started emitting shader-temps
properly, this causes issues where we end up with samplers and images as
shader-temps. That causes asserts while emitting them.

So let's remove the unused vars as well.

This fixes a piglit regression that somehow went unnoticed on CI.

Fixes: 85964945e7c ("zink: emit vars with nir_var_shader_temp mode")
Tested-by: Martin Roukala (né Peres) <martin.roukala@mupuf.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20227>