Document removal of CologneBlue & Modern from the tarball

Worth noting as a pre-upgrade thing, as trying to run the updater will fail
unless it's disabled or downloaded separately.

Reported by https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=909589.

Change-Id: I5ea6dc65d683dfceaa25104ca59f2f56c1ab1884