ir_to_mesa: Remove sat to clamp lowering pass

ir_to_mesa: Remove sat to clamp lowering pass

Fixes an infinite loop in swrast where the lowering pass unpacks saturate into
clamp but the opt_algebraic pass tries to do the opposite.

v3 (Ian):
This is a revert of commit cfa8c1cb "ir_to_mesa: lower ir_unop_saturate" on
the ir_to_mesa.cpp portion. prog_execute.c can handle saturates in vertex
shaders, so classic swrast shouldn't need this lowering pass.

Cc: "10.4" <mesa-stable@lists.freedesktop.org>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=83463
Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>