gnu: ghostscript: Switch to Artifex Ghostcript and update to 9.21.

gnu: ghostscript: Switch to Artifex Ghostcript and update to 9.21.

* gnu/packages/ghostscript.scm (ghostscript): Update to 9.21.
[source]: Fetch the source from Artifex.
[native-inputs]: Remove pkg-config.
[inputs]: Add jbig2dec and use libjpeg instead of libjpeg-8.
[arguments]: Add #:configure-flags.
[replacement]: Remove field.
(ghostcript/fixed): Remove variable.
* gnu/packages/patches/ghostscript-CVE-2017-8291.patch,
gnu/packages/patches/ghostscript-runpath.patch: Adjust to new upstream
source.
* gnu/packages/patches/ghostscript-CVE-2013-5653.patch,
gnu/packages/patches/ghostscript-CVE-2015-3228.patch,
gnu/packages/patches/ghostscript-CVE-2016-7976.patch,
gnu/packages/patches/ghostscript-CVE-2016-7978.patch,
gnu/packages/patches/ghostscript-CVE-2016-7979.patch,
gnu/packages/patches/ghostscript-CVE-2016-8602.patch: Delete files.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.