Remove stale function declaration.

It seems like the implementation for PDFiumEngine::RotateInternal() was
removed, but not the function declaration. This CL removes the
declaration.

Bug: n/a
Change-Id: I978d2173bc085b752af0710b754393a9b6b14e2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2119380Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: James MacLean <wjmaclean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753590}

pdf/pdfium/pdfium_engine.h

@@ -501,9 +501,6 @@ class PDFiumEngine : public PDFEngine,
   void OnSelectionTextChanged();
   void OnSelectionPositionChanged();
 
-  // Common code shared by RotateClockwise() and RotateCounterclockwise().
-  void RotateInternal();
-
   // Sets text selection status of document. This does not include text
   // within form text fields.
   void SetSelecting(bool selecting);