This article explains why unconfigured NTP server IP addresses are seen in the WAN edge config.

The following configuration is seen on the device, even if the "NTP servers" textbox in NTP is left blank.

set groups top system ntp server 216.239.35.0
set groups top system ntp server 216.239.35.4
set groups top system ntp server 216.239.35.8
set groups top system ntp server 216.239.35.12

This is expected and is a global config.

As we do not specify anything in the "NTP Servers" UI option, it will be blank. As a result, MIST pushes the default NTP servers for a seamless performance of the WAN edge devices.

If you want to delete this, you will have to delete these statements manually in the "Additional CLI commands" section.
delete groups top system ntp