spearmint: Don't change rgbGen of lightmap stages in nolightmap shaders

Restore Q3 behavior for lightmap stage in shader that specific surfaceParm nolightmap
(and does not also specify surfaceParm pointlight). Do not change it's rgbGen for diffuse light.

I think it was an unintended side affect of the ET code (comment says it's for q3map2
vertex-approximated surfaces). Official ET maps don't use $lightmap in nolightmap shaders.

Fixes a water shader near the beginning of Dark Salvation's darks01 map.