Remove wrong wxGrid::GetMainWindowOfCompositeControl() override

This method is supposed to be overridden in the sub-windows of a
composite control (and is indeed correctly implemented in
wxGridSubwindow), but it doesn't make any sense to implement it in the
parent window itself.

This method was probably never executed (which is how the problem went
unnoticed for 10+ years since 760be3f7cb),
but it's still wrong to define it here, so remove it.

include/wx/generic/grid.h

@@ -1947,8 +1947,6 @@ public:
 
 
     // override some base class functions
-    virtual wxWindow *GetMainWindowOfCompositeControl() wxOVERRIDE
-        { return (wxWindow*)m_gridWin; }
     virtual void Fit() wxOVERRIDE;
 
     // implementation only