Revert "Add quad-9 server"

This reverts commit 5adc8c3c.

test/pytest/test_dns.py

@@ -38,7 +38,7 @@ def test_dns(host):
         authorative_nameservers.append(ips[0].address)
 
     system_nameservers = dns.resolver.get_default_resolver()
-    external_nameservers = ['1.1.1.1', '8.8.8.8', '9.9.9.9']
+    external_nameservers = ['1.1.1.1', '8.8.8.8']
 
     results = []
     print('\nTesting authorative nameservers:')