Remove enforcement of 100% width on prettyphoto images

Forcing all images in a prettyphoto modal to be 100% width removes the ability for WC extension developers to use prettyphoto for inline content modals.
I am using it for a modal map for a WC extension, which broke < 768px because the tile images were 100% width.
This change makes images 100% width only when they are directly within the pp_full_res container, which is the behavior for image modals like the ones used by WC.