gnu: Remove unneeded libxml2 from native-inputs. (xmlto)

The native-search-paths from libxslt are reused within xmlto to provide
XML_CATALOG_FILES and SGML_CATALOG_FILES so it's unnecessary to include
libxml2 for the same effect.

* gnu/packages/wm.scm (i3status, awesome)[native-inputs]: Remove libxml2.
* gnu/packages/xml.scm (opensp)[native-inputs]: Ditto.

Signed-off-by: Maxim Cournoyer <maxim.cournoyer@gmail.com>

gnu/packages/wm.scm

@@ -342,7 +342,7 @@ (define-public i3status
      (list asciidoc
            perl
            pkg-config
-           docbook-xsl libxml2          ; for XML_CATALOG_FILES
+           docbook-xsl
            xmlto))
     (home-page "https://i3wm.org/i3status/")
     (synopsis "Status bar for i3bar, dzen2, xmobar or similar programs")
@@ -1226,7 +1226,6 @@ (define-public awesome
            doxygen
            gperf
            imagemagick
-           libxml2 ;for XML_CATALOG_FILES
            lua-ldoc
            pkg-config
            xmlto))

gnu/packages/xml.scm

@@ -1768,7 +1768,6 @@ (define-public opensp
     (native-inputs
      (list docbook-xml-4.1.2
            docbook-xsl
-           libxml2                      ;for XML_CATALOG_DIR
            xmlto
            ;; Dependencies to regenerate the 'configure' script.
            autoconf