[Minor refactoring] Removed unnecessary code from instance validator

lib/instance-validator.js

@@ -152,10 +152,6 @@ InstanceValidator.prototype.validate = function() {
     if (self.errors.length) {
       return new sequelizeError.ValidationError(null, self.errors);
     }
-
-    return new Promise(function(resolve) {
-      resolve();
-    });
   });
 };