textproc/libxml2: remove LICENSE block to unbreak libxml2-reference

The addition of the MIT licence block broke at least the
textproc/libxml2-reference port due to how the helper script
bsd.gnome-reference.mk works. Removed at the request of person that
suggested the license be added [2].

PR: 209806

PR: 212265 [2]
Reported by: cpm