This config no longer exists, so remove from docs.

This was changed to `no_subreaper` in
6e9f24b711 and, as far as I can tell,
`no_subreaper` doesn't exist as a config anymore.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

docs/getting-started.md

@@ -19,7 +19,6 @@ The daemon also uses a configuration file located in `/etc/containerd/config.tom
 A sample configuration file looks like this:
 
 ```toml
-subreaper = true
 oom_score = -999
 
 [debug]