Simplify sky fiber selection for subtractSky2d

The SubtractSky2dTask currently hard-wires use of TargetType.SKY fibers, but, SuNSS exposures don’t have any SKY fibers. The DummySubtractSky2dTask is much more configurable: it allows the user to specify what target types to use for sky, and allows filtering that list (e.g., odd fibers only). I propose to copy the mechanism to SubtractSky2dTask, which will allow users to test sky subtraction without the need to hack the pfsConfig beforehand.