Revert "Protect against non-determinstic sort"

This reverts commit 175bfe0482.

The change causes the following error if you're
unlucky enough to have such a marker column.

   ArgumentError: Only '=', '!=', 'is_()', 'isnot()' operators can be used with None/True/False

I saw test_floatingip_list_with_pagination failing,
where fixed_port_id of the marker was None.

Closes-Bug: #1656262
Change-Id: I6c32949d789f25d877d329ef0ae9d8650cb81acf

neutron/db/common_db_mixin.py

@@ -222,21 +222,6 @@ class CommonDbMixin(object):
         if sorts:
             sort_keys = db_utils.get_and_validate_sort_keys(sorts, model)
             sort_dirs = db_utils.get_sort_dirs(sorts, page_reverse)
-            # we always want deterministic results for subqueries so add pks
-            # to sorting when present.
-            # (http://docs.sqlalchemy.org/en/latest/orm/
-            #  loading_relationships.html#subqueryload-ordering)
-            # (http://docs.sqlalchemy.org/en/latest/faq/
-            #  ormconfiguration.html#faq-subqueryload-limit-sort)
-            uk_sets = sa_utils.get_unique_keys(model)
-            if uk_sets and limit:
-                # just grab first set of unique keys and use them.
-                # if model has no unqiue sets, 'paginate_query' will
-                # warn if sorting is unstable
-                for k in uk_sets[0]:
-                    if k not in sort_keys:
-                        sort_keys.append(k)
-                        sort_dirs.append('asc')
             collection = sa_utils.paginate_query(collection, model, limit,
                                                  marker=marker_obj,
                                                  sort_keys=sort_keys,