title: Delete source folder in PyDev project generates error "Source folder not found"
body: <p>When deleting a source folder in a PyDev project in the Project Explorer, the project PYTHONPATH is not updated and there is a red x marking in the outline with an error message saying "Source folder: [source folder name] not found".</p>
  
  <p>Is this really the intended behavior? I would expect the PYTHONPATH to be updated if source folders are removed. </p>
  
  <p>I'm using PyDev version 2.7.3 and Eclipse 3.7.2.</p>
  
  <p>Steps to repeat the problem:</p>
  
  <ol>
  <li>In Project Explorer, create a new PyDev project with File -> New -> PyDev Project</li>
  <li>Create a new PyDev source folder by right-clicking the new project and selecting New -> Source Folder (under the PyDev menu)</li>
  <li>Remove the source folder (which was just added) by right-clicking the source folder and select Delete.</li>
  <li>Add a new source folder (needed to trigger the error).</li>
  <li>There is a red x error mark and a message saying "Source folder: [source folder name] not found". </li>
  <li>Go to PyDev Properties. The deleted folder is still listed in the Source Folders tab under PyDev-PYTHONPATH.</li>
  </ol>
  
  <p>Note: When repeating the steps above in the Package Explorer instead of the Project Explorer, the red x's don't show up - so you can't see there is a problem. However, if you look in PyDev properties the behavior is the same - PYTHONPATH is not updated.</p>