meta-phosphor: libxml2: fix PACKAGECONFIG override

We intended to remove python from the class-target for libxml2, so
that we do not build the python bindings for target images.  There
have since been added additional PACKAGECONFIG values (namely ipv6),
so we shouldn't set the whole variable, but explicitly `:remove` it.

Change-Id: Idf8c4be33fe3216d2f81c6f585c31985dc8aafb4
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meta-phosphor/recipes-core/libxml2/libxml2_%.bbappend b/meta-phosphor/recipes-core/libxml2/libxml2_%.bbappend
index 97b68bc..42046b1 100644
--- a/meta-phosphor/recipes-core/libxml2/libxml2_%.bbappend
+++ b/meta-phosphor/recipes-core/libxml2/libxml2_%.bbappend
@@ -1,2 +1,2 @@
 # OpenBMC doesn't use python anymore, so no need for libxml python bindings
-PACKAGECONFIG:openbmc-phosphor:class-target = ""
+PACKAGECONFIG:remove:openbmc-phosphor:class-target = "python"