[image_picker] original file is leaking by resize.

[katout]

I discovered that the application size gets bigger every time I acquire an image with image_picker.

There are three reasons for this.

1. The quality setting of jpeg is bad. [image_picker] add resize quality #855
2. When resizing the taken picture, the original picture was not deleted. and The access path to that picture is in the dark...
3. I forgot to delete the picture acquired from native by flutter.

This pr is solve the second reason.

ps. I think that it need a code to delete the acquired picture in the sample code.

[cyanglaz]

Thank you very much for the contribution!

The same issue seems fixed in #1288
I am close the PR now.