Remove calls to wfLoadExtensionMessages()

wfLoadExtensionMessages() was officially deprecated in the MediaWiki
core in version 1.16 and is removed as of 1.21rc4.  This will make the
extension incompatible with MediaWiki 1.15 and earlier, but those
versions are no longer supported and the presence of these
functions will break the extension outright in MediaWiki 1.21
and later.