It was removed because it is illegal to distribute old jar files. I found a site where I was able to download all old files, from alpha to full release. If your just looking for the old classic lighting systen however, it's not that hard. Go into options->video settings->then go to smooth lighting and make it 0%. Btw, 1.7.3 doesn't have the default lighting system, it was made smooth at about 1.4 or 1.5.