st/nir: Drop setting interp mode on system values in builtins.

It's initialized to INTERP_MODE_NONE on creation, which makes more sense
for sysvals than FLAT, and is also the interp mode that GLSL IR sets up
for sysvals.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7320>
1 file changed