This reverts commit 9290c15ffc.
This was pinned during the upgrade to containerd 1.3 series, which
changed the default runtime on Linux to io.containerd.runc.v2.
No specific rationale was listed for this pinning, and clearly it's the
wrong thing to do in the presence of Windows, which does not have this
runtime.
Instead, we rely on the containerd-internal defaults, which distinguish
the runtimes for Linux and Windows.
Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>
156d66da02