When trying to delete the syslog server from the ATP UI under the detail page for the Management Appliance you are prompted with a yes/no question to delete it.  When you click on yes for delete it does nothing.

This issue is caused by having the syslog configured in the default configuration, but trying to delete it from the device details.  This is a bug, as the UI should not allow the user to delete the configuration if the "Use Default" checkbox is checked.  

This issue is fixed in the ATP 3.1 release build.  Please upgrade to this once the release build is available.  As a workaround until you can update, make sure to go to Settings-> Appliances-> Edit Default Appliance and delete it from there.