spearmint: Don't mark portalsky shaders as isSky

I made portalsky set isSky so that RDF_NOSKY would drop them.
Since then I made RDF_NOSKY drops surfaceParm sky (which all
FAKK's portalsky shaders have), so it's not needed.

Also the one stage in the portalsky shaders doesn't seem to draw
anything (blendFunc GL_ZERO GL_ONE).