gnu: libtiff: Update replacement to 4.0.8 [security fixes].

gnu: libtiff: Update replacement to 4.0.8 [security fixes].

See 'ChangeLog' in the source distribution for more information about
the bugs and security issues fixed in this release.

* gnu/packages/image.scm (libtiff)[replacement]: Replace with libtiff-4.0.8.
(libtiff/fixed): Replace with ...
(libtiff-4.0.8): New variable.
* gnu/packages/patches/libtiff-CVE-2017-7593.patch,
gnu/packages/patches/libtiff-CVE-2017-7594.patch,
gnu/packages/patches/libtiff-multiple-UBSAN-crashes.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Remove them.