Drop snap package (#2439)

The Clash snap package has not been a recommended way to use Clash for
quite some time, and it is a hassle to support.

In order to build a snap package, we build .deb packages for Clash with
Ubuntu 20.04 LTS. But the interaction between the Debian build system
and GHC is problematic, requiring significant effort to support and to
upgrade to a more recent Ubuntu release.

Additionally, snap packages have their own issues on distributions other
than Ubuntu. Given that we no longer recommend people use our snap
package and given the effort required to keep supporting it, we have
decided to drop the snap package.

(cherry picked from commit 070a6d0)

# Conflicts:
#	.ci/bindist/linux/debian/focal/buildinfo.json
#	.ci/bindist/linux/snap/snap/snapcraft.yaml
#	.ci/gitlab/publish.yml