libcontainer/user does not build at all on Windows any more, and this was breaking the client on Windows with upstream `runc`. As these functions are not used anyway, just split out and stop checking `runtime`. Signed-off-by: Justin Cormack <justin.cormack@docker.com> Upstream-commit: b7bd959294b5dc5b6e18b3fed5dde18f4f7f1d20 Component: engine