starttde: Correct the disposal of the TDE_FULL_SESSION variable

The variable is set and exported.  But previously it was unset before
xprop(1) can remove the property.  This commit corrects or gives the
xprop command the opportunity to remove the property (TDE_FULL_SESSION).

Signed-off-by: Matías Fonzo <selk@dragora.org>
(cherry picked from commit 5f0e42cda7)

starttde

@@ -839,8 +839,7 @@ for prefix in `echo "$exepath" | sed -n -e 's,/bin[^/]*/,/shutdown/,p'`; do
   done
 done
 
-unset TDE_FULL_SESSION
 xprop -root -remove TDE_FULL_SESSION
-unset TDE_SESSION_UID
+unset TDE_FULL_SESSION TDE_SESSION_UID
 
 echo "[starttde] Trinity shutdown complete." 1>&2