external-entity

chris steipp requested that:

* XMLReader must disable external entity loading before reading xml

i have included the corresponding libxml_disable_entity_loader() call and added a check
for an XMLReader::DOC_TYPE node in the XML document; if it exists, the extension will
delete the file and notify the user. we don’t need to accept XML metadata files with
<!DOCTYPE> declarations.

i also removed the outdated readXmlAsString method and its related calls and i18n messages.

Change-Id: If30609f2d51b7352ec09811f99f3bd857e142812