Set IOR to 1.5 in OSPRay
Most of dielectric materials have an IOR (except gemstones) between 1.33 and 1.58.
In PBR shader, it is set to 1.5 (F0 is 4%). For consistency, IOR is set to 1.5 in OSPRay now as well.
This MR also removes default irradiance that was added by error.