CI: Removed snap package generation

snapcraft version we use is no longer compatible with authentication
schemes snap store uses.  This could be fixed by updating the snapcraft
inside the docker image, but Ubuntu no longer ships an up to date
snapcraft version in their own repositories.  The other way to install
snapcraft is to manually fetch the project and core snaps just like we
do in https://code.videolan.org/videolan/docker-images/-/blob/master/vlc-ubuntu-focal/Dockerfile,
but that currently fails on Jammy due to conflict in Python versions
between what is shipped in Jammy and inside snapcraft project.

All in all, it seems snapcraft seems to be abandoned for our CI
use-case, and the usefulness of dav1d snap is disputable, so just drop
it altogether.  Packaging is still available in package/snap/ for the
brave souls who want to build it on their own.