xfce4-hdaps-dialogs.c: drop obsolete GTK_DIALOG_NO_SEPARATOR flag.
While creating our main dialog box, we used to pass in the
GTK_DIALOG_NO_SEPARATOR flag to omit the separator between the main
vbox and the action area. I no longer remember what real impact this
had, which is good I guess because that flag doesn't exist in gtk+-3.x.
Other XFCE panel plugins have fixed this by deleting the flag, and that
seems like a fine solution to me.