Avoid claiming unused aircraft

Probably the final Fix #97
Unused aircraft (assigned upon takeoff) would get claimed but since it's not possible to delete those flights after aborting, these flights wouldn't get released anymore. This should fix that issue, including a migrator change to correct the number of claimed aircraft per squadron.

qt_ui/windows/QWaitingForMissionResultWindow.py

@@ -1,7 +1,6 @@
 from __future__ import annotations
 
 import logging
-import os
 from pathlib import Path
 from typing import Optional
 
@@ -23,7 +22,6 @@ from jinja2 import Environment, FileSystemLoader, select_autoescape
 
 from game import Game
 from game.debriefing import Debriefing
-from game.persistency import base_path
 from game.profiling import logged_duration
 from qt_ui.simcontroller import SimController
 from qt_ui.windows.GameUpdateSignal import GameUpdateSignal