Revert "nixos/tests/networking: test nameservers via DHCP"

This reverts commit bad5251.

#348305 (comment)
Should've known that commit starting with `bad` will be no good.
Fixes nixosTests.networking.networkd.dhcpSimple
https://hydra.nixos.org/build/274843085/nixlog/8/tail

nixos/tests/networking/networkd-and-scripted.nix

@@ -132,10 +132,6 @@ let
               client.wait_until_succeeds("ip addr show dev enp2s0 | grep -q '192.168.2'")
               client.wait_until_succeeds("ip addr show dev enp2s0 | grep -q 'fd00:1234:5678:2:'")
 
-          with subtest("Wait until we have received the nameservers"):
-              client.wait_until_succeeds("grep -q 2001:db8::1 /etc/resolv.conf")
-              client.wait_until_succeeds("grep -q 192.168.2.1 /etc/resolv.conf")
-
           with subtest("Test vlan 1"):
               client.wait_until_succeeds("ping -c 1 192.168.1.1")
               client.wait_until_succeeds("ping -c 1 fd00:1234:5678:1::1")

nixos/tests/networking/networkmanager.nix

@@ -121,7 +121,6 @@ let
         static.wait_for_unit("NetworkManager.service")
 
         dynamic.wait_until_succeeds("cat /etc/resolv.conf | grep -q '192.168.1.1'")
-        dynamic.wait_until_succeeds("cat /etc/resolv.conf | grep -q '2001:db8::1'")
         static.wait_until_succeeds("cat /etc/resolv.conf | grep -q '10.10.10.10'")
         static.wait_until_fails("cat /etc/resolv.conf | grep -q '192.168.1.1'")
       '';

nixos/tests/networking/router.nix

@@ -72,7 +72,6 @@
           AdvSendAdvert on;
           AdvManagedFlag on;
           AdvOtherConfigFlag on;
-          RDNSS 2001:db8::1 {};
 
           prefix fd00:1234:5678:${toString n}::/64 {
             AdvAutonomous off;