Do not set global loading of DTD and entities, no more needed

The global settings have been replaced by the fine-grained
parameters passed to xmlReadFile.
Moreover a libxml2 regression prevents those parameters from
being used when the patch for CVE-2014-0191 is applied, see
https://bugzilla.gnome.org/show_bug.cgi?id=730290
A new libxml2 patch is going to be deployed soon, but anyway
this code works even without setting those parameters.

CCBUG: 335001