i965: Remove "disable_derivative_optimization" driconf option.

i965: Remove "disable_derivative_optimization" driconf option.

This was added in September 2013 when we first implemented the fast
(but lower quality) derivatives.  A quick Google search didn't turn
up anyone using or recommending the option, so I suspect no one does.

Applications that want to control the quality of their derivatives can
use the new GL_ARB_derivative_control extension, or use the glHint
mechanism.  The driconf option seems superfluous.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>