Allow purchasing air units whenever they can be built

core/src/com/unciv/logic/city/City.kt

@@ -420,7 +420,7 @@ class City : IsPartOfGameInfoSerialization, INamed {
         val tile = getCenterTile()
         return when {
             construction.isCivilian() -> tile.civilianUnit == null
-            construction.movesLikeAirUnits -> tile.airUnits.count { !it.isTransported } < 6
+            construction.movesLikeAirUnits -> return true // Dealt with in MapUnit.getRejectionReasons
             else -> tile.militaryUnit == null
         }
     }