Don't overwrite background in custom renderer in OSX/Cocoa.

Erasing background seems to be unnecessary and was actively harmful for the
selected items which didn't appear with the correct background colour. Just
don't do it at all.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62597 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/osx/cocoa/dataview.mm

@@ -1104,10 +1104,6 @@ outlineView:(NSOutlineView*)outlineView
 
     wxDataViewCustomRenderer * const renderer = obj->customRenderer;
 
-    // draw its own background:
-    [[self backgroundColor] set];
-    NSRectFill(cellFrame);
-
     wxDC * const dc = renderer->GetDC();
     renderer->WXCallRender(wxFromNSRect(controlView, cellFrame), dc, 0);
     renderer->SetDC(NULL);