Unref the sort_model so that the entire set of models will be properly

[David Hampton]

Log Message:
-----------
Unref the sort_model so that the entire set of models will
be properly deleted.

Tags:
----
gnucash-gnome2-dev

Modified Files:
--------------
    gnucash:
        ChangeLog
    gnucash/src/gnome-utils:
        gnc-tree-view-account.c