bridge: remove obsolete workaround

we no longer need to remove ipv6 routes from member interfaces when
adding to the bridge, because there will no longer be any ipv6 routes
on the member interfaces

modules/bridge/members.nix

@@ -24,7 +24,6 @@ let
         name = "${primary.name}.member.${member.name}";
         up = ''
           ip link set dev $(output ${member} ifname) master $(output ${primary} ifname)
-          ip -6 route flush dev $(output ${member} ifname)
         '';
         down = "ip link set dev $(output ${member} ifname) nomaster";
       };