containerd: Don't expose metrics by default

This can be turned on if required but we'll default to leaving it off
It's been enabled in the top-level linuxkit.yml
This port is also non-standard and there doesn't appear to be a standard
port for this as yet.

Signed-off-by: Dave Tucker <dt@docker.com>

pkg/containerd/etc/containerd/config.toml

@@ -13,4 +13,4 @@ subreaper = false
   level = "info"
 
 [metrics]
-  address = ":13337"
+  address = ""