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.