Don't remove bgcolor in selected markup in wxOSX

It does more harm than good: if an item uses background colors in the
markup, it stands to reason it's important and simply eliminating it
would result in confusing rendering. The decision how to best handle
background color in selection is application-specific.

src/osx/cocoa/dataview.mm

@@ -2948,7 +2948,6 @@ bool wxDataViewTextRenderer::MacRender()
         if ( [cell backgroundStyle] == NSBackgroundStyleDark )
         {
             [str removeAttribute:NSForegroundColorAttributeName range:NSMakeRange(0, [str length])];
-            [str removeAttribute:NSBackgroundColorAttributeName range:NSMakeRange(0, [str length])];
         }
 
         [cell setAttributedStringValue:str];