KEP-2033: KubeletInUserNamespace: drop unneeded label

It turned out that nodes do not need to have
`node.kubernetes.io/running-in-user-namespace: <BOOL>` labels.

Partially revert PR 5694 and PR 6114.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>