gnu: zziplib: Update to 0.13.68.

gnu: zziplib: Update to 0.13.68.

* gnu/packages/patches/zziplib-CVE-2017-5974.patch,
gnu/packages/patches/zziplib-CVE-2017-5975.patch,
gnu/packages/patches/zziplib-CVE-2017-5976.patch,
gnu/packages/patches/zziplib-CVE-2017-5978.patch,
gnu/packages/patches/zziplib-CVE-2017-5979.patch,
gnu/packages/patches/zziplib-CVE-2017-5981.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Remove them.
* gnu/packages/compression.scm (zziplib): Update to 0.13.68.
[source](patches): Remove.
[arguments]: Remove #:parallel-tests?.  Set #:tests? #f.