mu4e-shr2text does not anymore retrieve images.

When HTML emails contain references to remote images, retrieving these images
leaks informationo.  For example, the sender can see when I openend the email
and from which computer (IP address).  For this reason, it is preferrable to
not retrieve images.

See this discussion on mu-discuss: https://groups.google.com/forum/#!topic/mu-discuss/gr1cwNNZnXo