Removes unused XML files.

As we have stopped using Trello for training-guides and all of the cards are
ported to other XML files specific to books, this patch removes unused Trello
Card XML files.

Change-Id: Ibd6059a9917a37a2eedbd247f74584dce9b80922
Implements: blueprint separate-folders-for-books
Merge "Removes unused XML files."