Fix condition to conquer enemy asteroid when shield available

As building shields by enemies didn't fit in 16k version, there was a rare situation that shield was bought by player but the asteroid was taken in the same time by enemy. It should be rare but the bigger problem was that an enemy asteroid was not possible to be conquered because of the shield which cannot be destroyed by player, as this functionality was not implemented. However, function canAsteroidByConqueredByPlayerObject was checking the existence of asteroid shield. To resolve it, this condition was excluded from 16k version of the game, as shield cannot be build by enemy either.