ubuntukylin-keyring installed by default

Ubiquity installs the package ubuntukylin-keyring by default. We don't need or want this package installed by default.

On the other hand, currently we remove ubuntu-keyring from the list of packages we provide. It could be useful to modify those packages to install keys to /etc/apt/trusted.gpg.d/ubuntu-archive-*.gpg, similar to how it's done for the debian-archive-keyring (which neither we nor Ubuntu strip).