No fallback needed

Deleted the full size image as a fallback. One does not need the `src`
attribute in the markup, Picturefill dynamically adds it. This prevents
loading two images, the full size image and the optimized version. With
this change, only the appropriate version is loading which saves
bandwith. There is no need for a full size fallback image because
Picturefill handles this for you.

Tested in Chrome, Safari and Firefox. (Maybe the code needs to get
modified a little bit since I am not much into this and this may be
only a “hacky” way.)