remove unnecessary headers

Docker-DCO-1.1-Signed-off-by: Takahiro Maebashi <maebashi@iij.ad.jp> (github: maebashi)
namespaces: nsenter: ignore nonexistent namespaces

Docker-DCO-1.1-Signed-off-by: Takahiro Maebashi <maebashi@iij.ad.jp> (github: maebashi)