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.
This appears to be incompatible with pyinstaller. I get the following
when trying to run the executable generated with pyside6:
```
Traceback (most recent call last):
File "qt_ui\main.py", line 29, in <module>
File "PyInstaller\loader\pyimod03_importers.py", line 476, in exec_module
File "qt_ui\windows\QLiberationWindow.py", line 28, in <module>
File "PyInstaller\loader\pyimod03_importers.py", line 476, in exec_module
File "qt_ui\widgets\map\QLiberationMap.py", line 11, in <module>
ImportError: could not import module 'PySide6.QtPrintSupport'
```
