Revert "textproc/libxml2*: update to 2.15.1, split -core"
The design of this update was repeatedly rejected in the linked PR
with a prescribed different design. This commit is therefore
unauthorised.
This reverts commit 099bd713e09ebdd02d73fde770c50706f980404f.
With hat: desktop (textproc/libxml2), gnome (USES=gnome)
PR: 291316