gnu: Remove libwmf.

gnu: Remove libwmf.

This package contains many security vulnerabilities and is no longer maintained
upstream. See this discussion for more information:

https://lists.gnu.org/archive/html/guix-devel/2017-05/msg00478.html

* gnu/packages/image.scm (libwmf): Remove variable.
* gnu/packages/wv.scm (wv)[inputs]: Remove libwmf.
[arguments]: Remove field.
* gnu/packages/abiword.scm (abiword)[inputs]: Remove libwmf.
[source]: Remove patch 'abiword-wmf-version-lookup-fix.patch'.
* gnu/packages/patches/abiword-wmf-version-lookup-fix.patch,
gnu/packages/patches/libwmf-CAN-2004-0941.patch,
gnu/packages/patches/libwmf-CVE-2006-3376.patch,
gnu/packages/patches/libwmf-CVE-2007-0455.patch,
gnu/packages/patches/libwmf-CVE-2007-2756.patch,
gnu/packages/patches/libwmf-CVE-2007-3472.patch,
gnu/packages/patches/libwmf-CVE-2007-3473.patch,
gnu/packages/patches/libwmf-CVE-2007-3477.patch,
gnu/packages/patches/libwmf-CVE-2009-1364.patch,
gnu/packages/patches/libwmf-CVE-2009-3546.patch,
gnu/packages/patches/libwmf-CVE-2015-0848+CVE-2015-4588.patch,
gnu/packages/patches/libwmf-CVE-2015-4695.patch,
gnu/packages/patches/libwmf-CVE-2015-4696.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Remove them.