gnu: libxml2: Update to 2.14.6; deprecate python-libxml2.

Python bindings for libxml2 are now built as part of libxml2; remove all
python-libxml2 inputs for libxml2 (nearly all already had libxml2) and
deprecate the package.  This was done manually using

git grep python-libxml2 gnu/packages

to find affected packages and then check all their input fields.

With the update to libxml2, we can remove all the libxml2-next* variants.
This change was automated with:

git grep -rl libxml2-next | xargs sed -i 's/\(libxml2-next\|libxml2-next-for-grafting\)/libxml2/g'

* gnu/packages/xml.scm (libxml2): Update to 2.14.6.
[arguments]<#:phases>: Add configure-python phase (from python-libxml2).
[native-inputs]: Add pkg-config and python-minimal.
(libxml2-next, libxml2-next/fixed, libxml2-next-for-grafting): Remove.
(python-libxml2): Deprecate in favor of libxml2.

Change-Id: I2976f2fde086e9bc345d661edbdc125abbb7a753

gnu/packages/image-processing.scm

@@ -815,7 +815,7 @@ the OpenCV-Python library.")
            libpng
            (librsvg-for-system)
            libtiff
-           libxml2-next
+           libxml2
            libwebp
            matio
            openexr