gnu: Adjust asciidoc-related build options.

Commits 9099a457 and
3078821d make many package inputs and
build configurations related to asciidoc obsolete.

* gnu/packages/compton.scm (compton)[inputs]: Remove docbook-xml,
libxml2 and libxslt.
* gnu/packages/mail.scm (offlinemap)[native-inputs]: Remove libxslt.
* gnu/packages/linux.scm (btrfs-progs)[native-inputs]: Remove docbook-xml.
(perf)[inputs]: Remove docbook-xml and libxslt.
* gnu/packages/text-editors.scm (kakoune)[native-inputs]: Remove libxslt.
* gnu/packages/version-control.scm (cgit)[native-inputs]: Remove
docbook-xml, docbook-xsl, libxml2 and libxslt.
(cvs-fast-export)[native-inputs]: Remove docbook-xml, docbook-xsl,
libxml2 and libxslt.
* gnu/packages/web.scm (qutebrowser)[native-inputs]: Remove docbook-xml,
docbook-xsl, libxml2, and libxslt.
(tinyproxy)[native-inputs]: Remove docbook-xml, docbook-xsl, libxml2 and
libxslt.
* gnu/packages/wm.scm (awesome)[make-flags]: Remove XML_CATALOG_FILES flag.
(i3-wm)[inputs]: Remove docbook-xml.
[native-inputs]: Add libxml2.