メモリから assetToUnload をアンロードします
この関数はディスクに保管されたアセットに対してのみ呼び出しできます。
The referenced asset (assetToUnload) will be unloaded from memory. The object will become invalid and can't be loaded back from disk. Any subsequently loaded Scenes or assets that reference the asset on disk will cause a new instance of the object to be loaded from disk. This new instance will not be connected to the previously unloaded object.