Blender Git Loki

Cycles: Cleanup MultiGGX closure implementation

The implementation originally handled four different cases:
Regular glossy, glass, metallic fresnel glossy and diffuse.

However, only the first two are actually used currently. Therefore, this commit
removes the other two, which allows to simplify the code.

Additionally, due to the Principled BSDF, the function arguments are now
identical for glossy and glass, which allows to get rid of some ugly #ifdefs.