contrib: libxml2: update to 2.15.1

Some options are gone. Some options are off by default.
And we don't need the debug API.

contrib/src/libxml2/SHA512SUMS

@@ -1 +1 @@
-c1df7441c729430bdd2de52e9da69b171afc4b776e4a2eb43acbc71ada7a3814a465247fb5fca4c83ecf1c281f7745069ec3d051a7964cb7cc119a3d983f93ba  libxml2-2.11.4.tar.xz
+429c7d0dc3cbcb8de1bf6eb588fb8025fee009c9b25e87c96ecc5de57cc12b558b0ab8141f333d73ca687109335f297c9f9776abbb9da9afc9d8e79041c86b7f  libxml2-2.15.1.tar.xz

contrib/src/libxml2/rules.mak

@@ -1,7 +1,7 @@
 # libxml2
 
-LIBXML2_VERSION := 2.11.4
-LIBXML2_URL := https://download.gnome.org/sources/libxml2/2.11/libxml2-$(LIBXML2_VERSION).tar.xz
+LIBXML2_VERSION := 2.15.1
+LIBXML2_URL := https://download.gnome.org/sources/libxml2/2.15/libxml2-$(LIBXML2_VERSION).tar.xz
 
 PKGS += libxml2
 ifeq ($(call need_pkg,"libxml-2.0"),)
@@ -24,14 +24,9 @@ LIBXML2_CONF = \
         -DLIBXML2_WITH_XPATH=OFF \
         -DLIBXML2_WITH_XPTR=OFF \
         -DLIBXML2_WITH_MODULES=OFF \
-        -DLIBXML2_WITH_LEGACY=OFF \
         -DLIBXML2_WITH_ZLIB=OFF    \
         -DLIBXML2_WITH_ICONV=OFF   \
-        -DLIBXML2_WITH_HTTP=OFF    \
-        -DLIBXML2_WITH_FTP=OFF     \
         -DLIBXML2_WITH_REGEXPS=OFF \
-        -DLIBXML2_WITH_PYTHON=OFF \
-        -DLIBXML2_WITH_LZMA=OFF \
         -DLIBXML2_WITH_TESTS=OFF \
         -DLIBXML2_WITH_PROGRAMS=OFF