kernel: Revert "kernel: set root on NFS when enabled"

This reverts r48591, users needing NFS root will know how to configure
their platform and kernel command-line appropriately.

Reported-by: Petr Štetiar <ynezz@true.cz>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 48689