Fix config setup tool contact field to allow removal of empty non-URL fields

We just need to clear the binding when the value entered is completely empty

Fixes https://jira.coreos.com/browse/QUAY-815

static/js/core-config-setup.js

@@ -1114,6 +1114,10 @@ angular.module("core-config-setup", ['angularFileUpload'])
         var updateBinding = function() {
           if ($scope.value == null) { return; }
           var value = $scope.value || '';
+          if (!value) {
+            $scope.binding = '';
+            return;
+          }
 
           switch ($scope.kind) {
             case 'mailto':