Fix cast exception in AbstractProject.RemoveDocument

AbstractProject.RemoveDocument was trying to support removing
ContainedDocuments, but had a misplaced cast which negated the whole
attempt.

Fixes https://devdiv.visualstudio.com/DevDiv/_workitems/edit/776782

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

@@ -224,8 +224,6 @@ internal void AddDocument(IVisualStudioHostDocument document, bool isCurrentCont
         [Obsolete("This is a compatibility shim for TypeScript; please do not use it.")]
         internal void RemoveDocument(IVisualStudioHostDocument document)
         {
-            var shimDocument = (DocumentProvider.ShimDocument)document;
-
             var containedDocument = ContainedDocument.TryGetContainedDocument(document.Id);
             if (containedDocument != null)
             {
@@ -234,6 +232,7 @@ internal void RemoveDocument(IVisualStudioHostDocument document)
             }
             else
             {
+                var shimDocument = (DocumentProvider.ShimDocument)document;
                 VisualStudioProject.RemoveSourceFile(shimDocument.FilePath);
             }
         }