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:
0b14c2b67a, 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>