Remove excess cos theta terms from DisneyClearCoat::f()
Remove excess cos theta terms from DisneyClearCoat::f()

These are actually already present, via SmithG_GGX(). Empirically,
adding these also made the BxDF non-energy conserving.

(Issue #172).

mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nyue/pbrt-v3,nyue/pbrt-v3