We should remove format requirements from OPDS 2 image collection

[barmintor]

Copying from #80 (comment)

• The ability to display an image of a given format is an out-of-band client concern (and should not be a formal validation requirement of the feed)
• The images property/collection is not required for correctness
• The spec includes MIME information for the links for a client to decide whether it can make use of the link
• There's no difference for the reader between there not being a supported image and there not being images at all