title: clear constraints when using NSLayoutAnchor
body: <p>I'm new in creating views without interface builder .
  I'm using <code>NSLayoutAnchors</code> to create views!</p>
  
  <p>when I'm using some view that created in viewcontroller like this : </p>
  
  <pre><code>let borderView:UIView = {
  
      let view = UIView()
      view.translatesAutoresizingMaskIntoConstraints = false
      view.backgroundColor = UIColor.lightGray
      view.isUserInteractionEnabled = false
      view.alpha = 0.5
      return view
  
  }()
  </code></pre>
  
  <p>then i use this view to border sth in view like this : </p>
  
  <pre><code>view.addSubview(borderView)
  borderView.bottomAnchor.constraint(equalTo: headerView.bottomAnchor).isActive = true
  borderView.leftAnchor.constraint(equalTo: headerView.leftAnchor).isActive = true
  borderView.rightAnchor.constraint(equalTo: headerView.rightAnchor).isActive = true
  borderView.heightAnchor.constraint(equalToConstant: 1).isActive = true
  </code></pre>
  
  <p>then in another view i try this :</p>
  
  <pre><code>    informationView.addSubview(borderView)
  
      borderView.topAnchor.constraint(equalTo: informationView.topAnchor).isActive = true
      borderView.leftAnchor.constraint(equalTo: informationView.leftAnchor).isActive = true
      borderView.rightAnchor.constraint(equalTo: informationView.rightAnchor).isActive = true
      borderView.heightAnchor.constraint(equalToConstant: 1).isActive = true
  </code></pre>
  
  <p>but looks like this view has it's previous constraints end show constraints error ! </p>
  
  <p>how can I remove <code>borderView</code> constraints before reusing it ? </p>