spearmint: Don't make global fog affect blended surfaces

I made global fog affect sort > SS_OPAQUE to make ET foliage be fogged.
This has since been fixed by fogging sort <= SS_SEE_THROUGH (like ET).

Originally, I made blended surfaces only fog non-transparent areas
but that was removed because it broke fog block surface shaders.

Interestingly, SetIteratorFog for r_useGlFog was 'fixed' to not affect
blended surfaces. Making that change to software fog code corrects the
issues I'm seeing right now (ET radar rain splash and RTCW mp_castle
spider webs fog non-transparent areas), but it's better to just remove
the hack as I don't know of anything it improves.