title: can't delete site columns that aren't referenced
body: <p>I just created a couple of site columns and content types that reference them through VS2010.  I updated one of the fields and then tried to redeploy, but after retracting, deploy failed because the site columns previously created were still there.  I tried to delete them manually from the UI, and got an alert box with this message:</p>
  
  <blockquote>
    <p>Site columns which are included in content types cannot be deleted. 
    Remove all references to this site
    column prior to deleting it.</p>
  </blockquote>
  
  <p>I dug around in SharePoint Manager and didn't find any references, so I used powershell to enumerate all the content types and lists looking for references to my site columns and found nothing.  </p>
  
  <p>I tried to delete using PowerShell like this:</p>
  
  <pre><code>$web.Fields.Delete("StartTime")
  </code></pre>
  
  <p>which resulted in this error:</p>
  
  <pre><code>Exception calling "Delete" with "1" argument(s): "Site columns which are included in 
  content types or on lists cannot be deleted. Please remove all instances of this site 
  column prior to deleting it."
  At line:1 char:19
  + $web.Fields.Delete &lt;&lt;&lt;&lt; ("StartTime")
      + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
      + FullyQualifiedErrorId : DotNetMethodException
  </code></pre>
  
  <p>Finally, a simple inspection of the columns in PowerShell shows the following:</p>
  
  <pre><code>Title                               Id                                                         CanBeDeleted ParentList                                                       Sealed ListsFieldUsedIn                   
  -----                               --                                                         ------------ ----------                                                       ------ ----------------                   
  Start Time OBE                      6fa0d85b-9af1-408b-835f-d4c66536...                                True                                                                   False {}                                 
  Time Tracker Tags                   92bc866b-0415-45f0-b431-d4df69c4...                                True                                                                   False {}                                 
  </code></pre>
  
  <p>I'm experienced with MOSS 2007 and new to SP2010, but I've never seen this happen before.  Anyone have any hints?</p>