docs/papers: Remove duplicated papers/attacks-package-managers-ccs2008.pdf

This change is part is of tuf's commit:

Removed doc duplicate and an obsolete test module.

Author: Kon <[email protected]>
Origin: theupdateframework/python-tuf@e7c4352
Relate-to: theupdateframework/python-tuf#1380
URL: #159
Forwarded: #160
Signed-off-by: Philippe Coval <[email protected]>

Author: akonst