Delete old unused functions from core.process.filter

Everything that was happening here is probably better to just get from scipy and/or sci-kit image. In any event I am pretty sure no one uses these functions, they were untested, and in the case of background, actually been broken for a long time. We still want to background images, but in practice people just do im/bg or im-bg. We may want to put in a better backgrounding function at some point, but this broken one was not serving any purpose.