This removes the plugin section from the containerd configuration file
(`/var/run/docker/containerd/containerd.toml`) that is generated when
starting containerd as child process;

```toml
[plugins]
  [plugins.linux]
    shim = "containerd-shim"
    runtime = "runc"
    runtime_root = "/var/lib/docker/runc"
    no_shim = false
    shim_debug = true
```

This configuration doesn't appear to be used since commit:
0b14c2b, which switched the default runtime
to to io.containerd.runc.v2.

Note that containerd itself uses `containerd-shim` and `runc` as default
for `shim` and `runtime` v1, so omitting that configuration doesn't seem
to make a difference.

I'm slightly confused if any of the other options in this configuration were
actually used: for example, even though `runtime_root` was configured to be
`/var/lib/docker/runc`, when starting a container with that coniguration set
on docker 19.03, `/var/lib/docker/runc` doesn't appear to exist:

```console
$ docker ps
CONTAINER ID        IMAGE               COMMAND                  CREATED             STATUS              PORTS               NAMES
098baa4cb0e7        nginx:alpine        "/docker-entrypoint.…"   59 minutes ago      Up 59 minutes       80/tcp              foo

$ ls /var/lib/docker/runc
ls: /var/lib/docker/runc: No such file or directory

$ ps auxf
PID   USER     TIME  COMMAND
    1 root      0:00 sh
   16 root      0:11 dockerd --debug
   26 root      0:09 containerd --config /var/run/docker/containerd/containerd.toml --log-level debug
  234 root      0:00 containerd-shim -namespace moby -workdir /var/lib/docker/containerd/daemon/io.containerd.runtime.v1.linux/moby/09
  251 root      0:00 nginx: master process nginx -g daemon off;
  304 101       0:00 nginx: worker process
...

```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>