Remove 3rd PPA usage in the Magnum charm

[Ionut-Madalin Balutoiu]

When the Magnum charm was submitted, the Ussuri / Victoria distro packages needed several bug-fixes backported to be into a consistent state.

So, we ended up building new distro packages from latest stable Ussuri / Victoria branches into separate PPAs:
* https://launchpad.net/~openstack-charmers/+archive/ubuntu/magnum-ussuri
* https://launchpad.net/~openstack-charmers/+archive/ubuntu/magnum-victoria

That translated into a wrapper over the reactive install hook to enable PPAs first, and then execute the install hook:
https://opendev.org/openstack/charm-magnum/src/commit/8ad5c4fefec0c02c5c1feda8b38bdacfc2acfae5/src/lib/charm/openstack/magnum/magnum.py#L128-L146

This is supposed to be a workaround until Magnum packages are fixed into the cloud archive / default repositories. Remove the 3rd party Magnum PPA usage, once we don't need this workaround.