demos: Simplify popovers demo

Don't do much unconventional stuff here. This is a demo, not a test.

https://bugzilla.gnome.org/show_bug.cgi?id=767851

demos/gtk-demo/popover.c

@@ -51,10 +51,6 @@ create_complex_popover (GtkWidget       *parent,
   popover = create_popover (parent, content, GTK_POS_BOTTOM);
   g_object_unref (content);
 
-  gtk_widget_set_margin_start (popover, 10);
-  gtk_widget_set_margin_end (popover, 10);
-  gtk_widget_set_margin_bottom (popover, 10);
-
   return popover;
 }