Revert "stdenv/setup.sh: unbreak *.lz sources on darwin"

This reverts commit fcafdd27616b91049c506441802b9cfe970388d4. We're
trying to modularise stdenv, not add more ad-hoc compression support.

pkgs/stdenv/generic/setup.sh

···
             # Don't rely on tar knowing about .xz.
             xz -d < $curSrc | tar xf -
             ;;
-        *.tar.lz )
-            # Don't rely on tar knowing about .lz.
-            lzip -d < $curSrc | tar xf -
-            ;;
         *.tar | *.tar.* | *.tgz | *.tbz2)
             # GNU tar can automatically select the decompression method
             # (info "(tar) gzip").