Remove all covers, they are autogenerated now.
Thanks to a long effort to find a way to remove this duplication (having two time to enter the title; on cover + on top title). And special thanks to Ctk and Midgard for starting the investigation, and mainly thanks to @gunchleoc for the creation of all the tools and all the tests for making this big change possible. Bravo! Goodbye to 6 years of covers... Note: the cover that required special translation will be replaced with new artworks as mentionned on the issue. Ref: #86