SA_CUSTOM seems unused

@gpichier @elena : since there is a weird and dangerous connection between StarAccretion and Viscosity (using if/else combination that do not always covers all cases in a reassuring ay), I'm trying to remove any unused code.

Right now, SA_CUSTOM seems useless as it specify the same values, cnfig wise, as SA_CONSTANT and is never used in the non config code.

Do you confirm we can prune it ?

Thanks