Dispose the HostDocument from the misc project when we stop tracking the file.

src/VisualStudio/Core/Def/Implementation/ProjectSystem/MiscellaneousFilesWorkspace.cs

@@ -350,6 +350,8 @@ private void DetachFromDocument(uint docCookie, string moniker)
                 _hostProjects.Remove(hostProject.Id);
                 _docCookiesToHostProject.Remove(docCookie);
 
+                document.Dispose();
+
                 return;
             }
         }