Remove unused IsCanvasWindow() from wxUniv

This function existed but was never used, so just remove it.

include/wx/univ/panel.h

@@ -29,8 +29,6 @@ public:
         Create(parent, winid, pos, size, style, name);
     }
 
-    virtual bool IsCanvasWindow() const { return true; }
-
 private:
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxPanel);
 };

include/wx/univ/window.h

@@ -140,10 +140,6 @@ public:
     // methods used by wxColourScheme to choose the colours for this window
     // --------------------------------------------------------------------
 
-    // return true if this is a panel/canvas window which contains other
-    // controls only
-    virtual bool IsCanvasWindow() const { return false; }
-
     // return true if this control can be highlighted when the mouse is over
     // it (the theme decides itself whether it is really highlighted or not)
     virtual bool CanBeHighlighted() const { return false; }