Drop ImageCursorSource

The ImageCursorSource used to be primarily a porting aid. That is, if we
couldn't port some code to SurfaceCursorSource or ShapeCursorSource, the
ImageCursorSource was used in interim. Now, all parts of kwin have been
ported to ShapeCursorSource and SurfaceCursorSource, so the image cursor
source can be dropped.