turnip: drop GS clear path

turnip: drop GS clear path

We didn't know how to write layer id without GS, since that's the only way
to do it through VK/GL, and the blob didn't implement this clear case (and
failed cases where it was absolutely necessary). However now we know how to
set it after some educated guesses and looking at tess/geom traces, so the
GS path can be dropped.

Signed-off-by: Jonathan Marek <jonathan@marek.ca>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5790>