Revert "util-linux: add nsenter option"

This reverts commit 9f91d796.

Not all toolchains have setns(), and the configure script automatically
enables nsenter if it does, so get rid of the explicit option.

See http://lists.busybox.net/pipermail/buildroot/2014-February/088669.html


for details.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>