main/gtk-doc: remove obsolete py-libxml2 dependency

This package does no longer exist and is apparently not needed.

main/gtk-doc/APKBUILD

@@ -7,7 +7,7 @@ url="https://www.gtk.org/gtk-doc/"
 arch="noarch"
 license="GPL-2.0-or-later AND GFDL-1.1-or-later"
 depends="docbook-xsl python3 py3-six pkgconf glib-dev py3-pygments"
-makedepends="rarian-dev py-libxml2 gettext autoconf automake libtool"
+makedepends="rarian-dev gettext autoconf automake libtool"
 checkdepends="bc"
 source="https://download.gnome.org/sources/gtk-doc/$pkgver/gtk-doc-$pkgver.tar.xz"