overlay coreos: Drop obsolete subreaper key from containerd config

[krnowak]

This key was first renamed to no_subreaper in 2017, then eventually dropped. Now containerd prints a warning like:

time="2025-04-08T22:59:42Z" level=warning msg="Ignoring unknown key in TOML" column=1 error="strict mode: fields in the document are missing in the target struct" file=/usr/share/containerd/config.toml key=subreaper row=8

There seem to be no replacement for it, so just drop it.