title: doctrine2 remove ManyToMany relationship
body: <p>I have a Group Entity with the following class member and mapping:</p>
  
  <pre><code>/**
   * @ManyToMany(targetEntity="Group", cascade={"persist"})
   * @JoinTable(name="groups_children",
   *      joinColumns={@JoinColumn(name="group_id", referencedColumnName="id")},
   *      inverseJoinColumns={@JoinColumn(name="child_id", referencedColumnName="id", unique=true)}
   *      )
   */
  private $children;
  </code></pre>
  
  <p>Adding a child is trivial:</p>
  
  <pre><code>public function addChild(Group $group)
  {
      if (! $this-&gt;hasChild($group)) {
          $this-&gt;children[] = $group;
          App::getEntityManager()-&gt;persist($this);
      }
  }
  </code></pre>
  
  <p>Removing a child:</p>
  
  <pre><code>???????
  </code></pre>
  
  <p>How would I remove a child?</p>