Fix bosses doing actions in on_created()

[christopho]

Before Solarus 1.2, bosses had a special case: they were initially disabled. For them, on_created() was called later, when enabling the enemy.

In Solarus 1.2, enemy:on_created() is called at the entity creation, and bosses are no longer implicitly disabled, which is the proper way to do. Therefore, bosses whose on_created() event do things like creating other enemies should be fixed.