[GTK] Remove orphaned code

Changes were made to sidebar theming in commit 5a6f202 and this code was
forgotten to be removed.

deluge/ui/gtk3/filtertreeview.py

@@ -100,11 +100,6 @@ class FilterTreeView(component.Component):
 
         self.treeview.connect('button-press-event', self.on_button_press_event)
 
-        # colors using current theme.
-        style_ctx = component.get('MainWindow').window.get_style_context()
-        self.colour_background = style_ctx.get_background_color(Gtk.StateFlags.NORMAL)
-        self.colour_foreground = style_ctx.get_color(Gtk.StateFlags.NORMAL)
-
         # filtertree menu
         builder = Gtk.Builder()
         builder.add_from_file(