Fix: session.xsd is already present in dist_xml_DATA

There is no need to include session.xsd as part of xml_DATA since
dist_xml_DATA will already ensure it is part of the dist target and
installed.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>