Revert "XZ utils is required by tar"

This reverts commit ce265a569c5e8d32de3b8ea78aa16abf68a962c7.

xz-util should be xz-utils here, and according to this list, http://releases.ubuntu.com/releases/trusty/ubuntu-14.04-desktop-amd64.manifest, it's already in it, since we bumped the ubuntu version, we don't need to install it by ourselves anymore.