lister.core.db_utils: Remove dead code

swh/lister/core/db_utils.py

-# Copyright (C) 2015  Stefano Zacchiroli <zack@upsilon.cc>
-# License: GNU General Public License version 3, or any later version
-# See top-level LICENSE file for more information
-
-from contextlib import contextmanager
-
-
-@contextmanager
-def session_scope(mk_session):
-    session = mk_session()
-    try:
-        yield session
-        session.commit()
-    except:  # noqa
-        session.rollback()
-        raise
-    finally:
-        session.close()