gnu: crypto++: Update to 6.0.0.

* gnu/packages/crypto.scm (crypto++): Update to 6.0.0.
* gnu/packages/patches/crypto++-fix-dos-in-asn.1-decoders.patch: Delete
file.
* gnu/local.mk (dist_patch_DATA): Remove it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>