title: Unregister browser layer
body: <p>In one Plone site I removed one product but the it's browser layer keeps registered (raising some exceptions in the log)</p>
  
  <pre><code>2015-09-10T06:00:32 WARNING OFS.Uninstalled Could not import class 'ICollectiveMailchimp' from module 'collective.mailchimp.interfaces'
  </code></pre>
  
  <p>I tried to remove this layer using the <code>plone.browserlayer.utils.unregister_layer</code>, but it send me the <code>KeyError</code> exception:</p>
  
  <pre><code>&gt;&gt;&gt; pprint(utils.registered_layers())
  [.
   .
   .
   &lt;InterfaceClass collective.mailchimp.interfaces.ICollectiveMailchimp&gt;,
   &lt;InterfaceClass collective.liveblog.interfaces.IBrowserLayer&gt;,
   &lt;InterfaceClass collective.blueline.interfaces.IBrowserLayer&gt;]
  &gt;&gt;&gt; layer = utils.registered_layers()[-3]
  &gt;&gt;&gt; layer
  &lt;InterfaceClass collective.mailchimp.interfaces.ICollectiveMailchimp&gt;
  &gt;&gt;&gt; layer.__name__
  'ICollectiveMailchimp'
  &gt;&gt;&gt; utils.unregister_layer(layer)
  Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
    File "/home/cartacapital/cartacapital.portal.buildout/eggs/plone.browserlayer-2.1.3-py2.7.egg/plone/browserlayer/utils.py", line 38, in unregister_layer
      raise KeyError("No browser layer with name %s is registered." % name)
  KeyError: 'No browser layer with name &lt;InterfaceClass collective.mailchimp.interfaces.ICollectiveMailchimp&gt; is registered.'
  &gt;&gt;&gt; utils.unregister_layer(layer.__name__)
  Traceback (most recent call last):
    File "&lt;console&gt;", line 1, in &lt;module&gt;
    File "/home/cartacapital/cartacapital.portal.buildout/eggs/plone.browserlayer-2.1.3-py2.7.egg/plone/browserlayer/utils.py", line 38, in unregister_layer
      raise KeyError("No browser layer with name %s is registered." % name)
  KeyError: 'No browser layer with name ICollectiveMailchimp is registered.'
  </code></pre>
  
  <p>do I need to register again this layer before unregister it? </p>