Signals that this DownloadHandler is no longer being used, and should clean up any resources it is using.
DownloadHandler オブジェクトのを使用完了後、このメソッドを呼び出す必要があります。
For convenience, UnityWebRequest exposes the UnityWebRequest.disposeDownloadHandlerOnDispose property, which will automatically call this method when UnityWebRequest.Dispose is called.
If you elect not to use UnityWebRequest.disposeDownloadHandlerOnDispose (by setting it to false), then you must call Dispose on the DownloadHandler yourself. This should only be in rare cases, such as when you wish to save the data in the DownloadHandler's internal buffer(s) for later processing.