virtualx: remove GLUtesselator forward declarations from TVirtualX.h

It is not used in TVirtualX interfaces.
It is re-declared in TGLUtil.h include.
Was introduced 14 years ago with the commit:

https://github.com/root-project/root/commit/fc7ab2b

In my mind - it is wrong place

core/base/inc/TVirtualX.h

@@ -49,11 +49,6 @@ enum ECursor { kBottomLeft, kBottomRight, kTopLeft, kTopRight,
 class TPoint;
 class TString;
 class TGWin32Command;
-#ifdef __cplusplus
-   class GLUtesselator;
-#else
-   extern "C" { struct GLUtesselator; }
-#endif
 
 class TVirtualX : public TNamed, public TAttLine, public TAttFill, public TAttText, public TAttMarker {