efl_ui_slider: this does not use anything from Efl.Content

Reviewed-by: Cedric BAIL <cedric.bail@free.fr>
Reviewed-by: Xavi Artigas <xavierartigas@yahoo.es>
Differential Revision: https://phab.enlightenment.org/D7783

src/lib/elementary/efl_ui_slider.eo

@@ -1,7 +1,6 @@
 class Efl.Ui.Slider extends Efl.Ui.Layout implements Efl.Ui.Range_Interactive, Efl.Ui.Direction,
                      Efl.Access.Value,
-                     Efl.Access.Widget.Action,
-                     Efl.Content
+                     Efl.Access.Widget.Action
 {
    [[Elementary slider class]]
    legacy_prefix: null;