headerbar: Don't change the custom title's valign

This allows custom titles to use the vertical alignment of their choice
without having to set it manually after it has been added to the header
bar, hence allowing them to define their preferred alignment from a UI
template.

This is useful for widgets like the view switcher which requires filling
the header bar's whole height.

gtk/gtkheaderbar.c

@@ -1356,7 +1356,6 @@ gtk_header_bar_set_custom_title (GtkHeaderBar *bar,
 
       gtk_header_bar_reorder_css_node (bar, GTK_PACK_START, priv->custom_title);
       gtk_widget_set_parent (priv->custom_title, GTK_WIDGET (bar));
-      gtk_widget_set_valign (priv->custom_title, GTK_ALIGN_CENTER);
 
       if (priv->label_box != NULL)
         {