i965: Delete previous workaround for textureGrad with shadow samplers.

i965: Delete previous workaround for textureGrad with shadow samplers.

It had many problems:
- The shadow comparison was done post-filtering.
- It required state-dependent recompiles whenever the comparison
  function changed.
- It didn't even work: many cases hit assertion failures.
- I never implemented it for the VS.

The new lowering pass which converts textureGrad to textureLod by
computing the LOD value works much better.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>