remove IPv6 docker

Will re-submit when ICMP dest unreachable is handled correctly and it can be
turned back on.

PiperOrigin-RevId: 321227330

tools/vm/ubuntu1604/30_docker.sh

@@ -54,11 +54,8 @@ while true; do
 done
 
 # Enable experimental features, for cross-building aarch64 images.
-# Enable Docker IPv6.
 cat > /etc/docker/daemon.json <<EOF
 {
-    "experimental": true,
-    "fixed-cidr-v6": "2001:db8:1::/64",
-    "ipv6": true
+    "experimental": true
 }
 EOF