Fiber trace throughput should not be applied to 2d sky psfs

2d sky subtraction at the moment applies throughput to the created psf, deduced from fiber trace (https://github.com/Subaru-PFS/drp_stella/blob/master/python/pfs/drp/stella/subtractSky2d.py#L81-L108). As discussed on Monday Princeton telecon on Aug 16, it should only apply the terms related to quantum efficiency. As a first step, it might be sufficient to just approximate these terms as unity over the small area that in-focus psf occupies.