バンドル内のすべてのアセットをアンロードします。
アンロードすることでアセットバンドルの中にあるオブジェクトに関連するメモリをすべて解放します。
When unloadAllLoadedObjects is false, compressed file data for assets inside the bundle will be unloaded, but any actual objects already loaded
from this bundle will be kept intact. Of course you won't be able to load any more objects from this bundle.
When unloadAllLoadedObjects is true, all objects that were loaded from this bundle will be destroyed as well. If there are game
objects in your scene referencing those assets, the references to them will become missing.