[credentialsmanager] Remove redundant tests

third_party/WebKit/LayoutTests/http/tests/credentialmanager/resources/interfaces.js

-function verify_interface(name, instance, attributes) {
-  assert_true(name in self,
-              name + ' should be an defined type');
-  if (instance) {
-    assert_true(instance instanceof self[name],
-                instance + ' should be an instance of ' + name);
-    Object.keys(attributes || {}).forEach(function(attribute) {
-        var type = attributes[attribute];
-        assert_true(attribute in instance,
-                    attribute + ' should be an attribute of ' + name);
-        assert_equals(typeof instance[attribute], type,
-                      attribute + ' should be of type ' + type);
-      });
-  }
-}