xwayland: Don't queue frame callbacks when role assigned
'xwayland: Do not queue frame callbacks unconditionally' changed the frame callback behavior of Xwayland surfaces so that they behave the same way as other actor surfaces (e.g. xdg-shell ones), except for the case when they are initially assigned.
Remove this special casing as well including the now incorrect comment, so that the Xwayland surfaces behave the same as the others in this regard also when assigning.