Remove 'Image.extents' field
Remove 'Image.extents' field

This field is superseded by 'dimensions'. All Image documents in the
database have been updated to remove the 'extents' field, after ensuring
that 'dimensions' contained equivalent data.

SlideAtlas/SlideAtlas-Server,SlideAtlas/SlideAtlas-Server,SlideAtlas/SlideAtlas-Server,SlideAtlas/SlideAtlas-Server