title: Remove custom spacing in UIStackView
body: <p>I have a stackview with three subviews:</p>
  
  <pre><code>stackView.spacing = 8.0
  
  stackView.addArrangeSubview(view1)
  stackView.addArrangeSubview(view2)
  stackView.addArrangeSubview(view3)
  </code></pre>
  
  <p>At some point, I add custom spacing after view2:</p>
  
  <pre><code>stackView.setCustomSpacing(24.0, after: view2)
  </code></pre>
  
  <p>Now, I want to remove the custom spacing. Is this possible using UIStackView? The only way I can think of is</p>
  
  <pre><code>stackView.setCustomSpacing(8.0, after: view2)
  </code></pre>
  
  <p>but this will break if I ever change the spacing of my stackView to something other than 8.0, because spacing will remain 8.0 after view2.</p>