Hide the border of the completionwidget

Having a light Qt theme but a dark qutebrowser theme, one can see an
ugly white border around the completion widget which is some relict from
the underlying Qt widget QTreeView. As qutebrowser has its own theming
settings for the mainwindow, it should hide the Qt theme as far as
possible.

qutebrowser/completion/completionwidget.py

@@ -59,6 +59,7 @@ class CompletionView(QTreeView):
             {{ color['completion.bg'] }}
             alternate-background-color: {{ color['completion.alternate-bg'] }};
             outline: 0;
+            border: 0px;
         }
 
         QTreeView::item:disabled {