radeonsi: remove a gfx11 check in si_shader_gs (legacy GS)

Gfx11 doesn't support legacy GS.

Reviewed-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21525>
1 file changed