Remove devmapper snapshotter

[mxpv]

The device mapper snapshotter upstreamed to containerd (and released in 1.3), it's more up to date and there are no reasons to keep two versions of the same code. I think it's time to remove it from our repo and rely on containerd.
Plan:

• Add a guide how to create a thin-pool (Modify getting-started to use devmapper snapshottter #317)
• Update docs (Use devmapper snapshotter in quickstart guide #194, Update quickstart #361)
• Update CI
• Remove code packages (Remove snapshotter package #344)
• Remove naive snapshotter switchers from examples and integration tests (Remove mention of naive snapshotter from examples and tests #366)

[kzys]

The CI (BuildKite) is actually using the upstreamed snapshotter.