[Image Redux] Remove unused Image::sourceSize() and BitmapImage::sourceSize()

https://bugs.webkit.org/show_bug.cgi?id=326350

Reviewed by Simon Fraser.

Removes unused Image::sourceSize() and BitmapImage::sourceSize().

* Source/WebCore/platform/graphics/BitmapImage.h:
* Source/WebCore/platform/graphics/Image.cpp:
* Source/WebCore/platform/graphics/Image.h:

Canonical link: https://commits.webkit.org/322723@main