Try this
CREATE OR REPLACE FUNCTION drop_table(name TEXT)
RETURNS void AS
$BODY$
DECLARE statement TEXT;
BEGIN
statement := 'DROP TABLE ' || name;
EXECUTE statement;
END;
$BODY$
LANGUAGE plpgsql VOLATILE SECURITY DEFINER
COST 100;
SELECT drop_table(relname)
FROM pg_stat_user_tables
WHERE n_live_tup = 0;