After using `networksetup -setdnsservers Wi-Fi "8.8.8.8" "8.8.4.4"` how do I clear them?
If you're concerned only about cleaning DNS entries, the  command help gives you instructions what to do:

Usage: networksetup -setdnsservers &lt;networkservice&gt; &lt;dns1&gt; [dns2] [...]
       Set the &lt;networkservice&gt; DNS servers to &lt;dns1&gt; [dns2] [...]. Any 
       number of dns servers can be specified. Specify "Empty" for &lt;dns1&gt; 
       to clear all DNS entries.


In my case

sudo networksetup -setdnsservers Wi-Fi "Empty"