The peerdns settings makes little practical sense for proto:static
interfaces, so revert to allow setting the DNS server list directly.
Fixes: faad7464a8 ("luci-mod-network: add support for network.device sections")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit 2bfd4908a9cfeac74c7abc31d8cef0bf1e58af52)