'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.

GNOME/mutter!964