softpipe: Remove unused static function

Thanks to Eric Engestrom for pointing out that there was something wrong
with that function.

Fixes: 724a7350
  softpipe: Prepare handling explicit gradients

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
(cherry picked from commit 9c611fb3)