[Merge] ~ddstreet/livecd-rootfs:remove-software-properties-dep into livecd-rootfs:ubuntu/master

[Dan Streetman]

Review: Disapprove

> an alternate option is https://code.launchpad.net/~ddstreet/livecd-rootfs/+git
> /livecd-rootfs/+merge/389260

alternative was merged, so this no longer needed
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https://code.launchpad.net/~ddstreet/livecd-rootfs/+git/livecd-rootfs/+merge/389235
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