Remove XML::Simple use

According to XML::Simple status, usage of this module is strongly discouraged:

The use of this module in new code is strongly discouraged. Other modules are available which provide more straightforward and consistent interfaces. In particular, XML::LibXML is highly recommended and you can refer to Perl XML::LibXML by Example for a tutorial introduction.

XML::Twig is another excellent alternative.

The major problems with this module are the large number of options (some of which have unfortunate defaults) and the arbitrary ways in which these options interact - often producing unexpected results.

In LLNG, it is used here:

• lemonldap-ng-portal/lib/Lemonldap/NG/Portal/Lib/CAS.pm
• lemonldap-ng-portal/lib/Lemonldap/NG/Portal/Lib/SAML.pm