Don't change the clipboard in wxNotebook::SetPageImage() in wxOSX.

For some reason the implementation of this method (see #12754) also put the
image on the clipboard, which was completely unexpected and unnecessary, so
just don't do it.

Closes https://github.com/wxWidgets/wxWidgets/pull/77

src/osx/cocoa/notebook.mm

@@ -161,12 +161,6 @@
     m_image = image;
     if(!m_image)
         return;
-    [[NSPasteboard generalPasteboard]
-        declareTypes:[NSArray arrayWithObject:NSTIFFPboardType]
-        owner:nil];
-    [[NSPasteboard generalPasteboard]
-        setData:[m_image TIFFRepresentation]
-        forType:NSTIFFPboardType];
 }
 @end // implementation WXCTabViewImageItem : NSTabViewItem