title: Destroy cascade is not working in Sequelize orm
body: <p>I have two models created with Sequelize cli; first model is <code>user_number</code>, the second model is <code>emailSign</code>.</p>
  
  <p>This is my <code>user_number</code> model:</p>
  
  <pre><code>    'use strict';
       module.exports = function(sequelize, DataTypes) {
       var user_number = sequelize.define('user_number', {
       user_id: DataTypes.INTEGER,
       callRoute_id: DataTypes.INTEGER,
       password: DataTypes.STRING,
       }, {
       underscored: true,
       classMethods: {
           associate: function(models) {
              // associations can be defined here
               user_number.hasMany(models.emailSign, {
                  foreignKey: 'user_number_id',
                  onDelete: "CASCADE",
               });
             }
             }
          });
        return user_number;
      };
  </code></pre>
  
  <p>This is my <code>emailSign</code> model:</p>
  
  <pre><code>      'use strict';
        module.exports = function(sequelize, DataTypes) {
        var emailSign = sequelize.define('emailSign', {
        email: DataTypes.STRING,
        user_number_id: DataTypes.INTEGER
        }, {
           underscored: true,
           classMethods: {
               associate: function(models) {
               // associations can be defined here
               emailSign.belongsTo(models.user_number, {
               foreignKey: 'user_number_id',
  
             })
           }
          }
        });
        return emailSign;
            }; 
  </code></pre>
  
  <p>I want destroy <code>user_number</code> to cascade remove <code>emailSign</code>. My code for destroy is</p>
  
  <pre><code>User_number.destroy({where:{id : 5}}).then(function(){
  
  resp.status(200);
  ) 
  </code></pre>
  
  <p><code>user_number</code> with id 5 is removed, but <code>emailSign</code> is not removed.</p>
  
  <p>How can I remove the related <code>emailSign</code> as well?  </p>
  
  <p>My Sequelize version is 3.30.4 </p>