After this change, players will always have the final say in what
missions a squadron can be assigned to. Squadrons are not able to
influence the default auto-assignable missions either because that
property is always overridden by the campaign's air wing configuration
(the primary and secondary task properties). The `mission-types` field
of the squadron definition has been removed since it is no longer
capable of influencing anything. I haven't bothered cleaning up the now
useless data in all the existing squadrons though.
Fixes https://github.com/dcs-liberation/dcs_liberation/issues/2785.
94b8aa7213