mesa: Kill gl_fragment_program::FogOption with fire

mesa: Kill gl_fragment_program::FogOption with fire

All drivers expect this to always be GL_NONE.  Don't let there be any
opportunity for a bad value to leak out and infect some unsuspecting
driver.  If any driver for hardware that had fixed-function
per-fragment fog (i915 and perhaps some r300-ish) was ever going to
add support, it would have done it by now.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Corbin Simpson <MostAwesomeDude@gmail.com>
Acked-by: Alex Deucher <alexdeucher@gmail.com>