luci-mod-network: remove unused dns_search property

luci-mod-network: remove unused dns_search property

The same functionality is fulfilled by the "domain" property. Two fields which
have the same semantics and purpose. None of odhcpd, dnsmasq or odhcp6c use the
dns_search property.

See also:
https://github.com/openwrt/odhcpd/issues/206

The local device might anyway use the "search" entries from /etc/resolv.conf at
the lower layers, but IPv6 RA and DHCPv6 is controlled by the "domain" property,
which is distributed to clients.

Signed-off-by: Paul Donald <[email protected]>