No more .bz2 files from kernel.org

Providing every release in .tar.gz, .tar.bz2 and .tar.xz was deemed overly redundant. The goal is to provide two formats -- .tar.gz for fast decompression and maximum compatibility between various platforms, and .tar.xz for smaller tarballs (in order to save bandwidth for people downloading releases). With xz becoming ubiquitous, continuing to provide .bz2 releases was seen as offering little ongoing value, and so was dropped.