feat: Remove jsonnet-tool checksum command as jsonnet itself has a similar jsonnet-deps

This command was originally introduced in !57 (merged) in order to facilitate caching in gitlab-com/runbooks!5618 (merged).

It turns out however that jsonnet itself already ships with a very similar jsonnet-deps command.

That makes the checksum command obsolete, as we can replace it with:

jsonnet-deps -J . -J ../libsonnet [...] | xargs sha256sum

Thus we are now removing it in favour of what jsonnet ships with out of the box.