No temporary tables in db_query_range()

The comment for db_query_range() in database.mysqli.inc and database.mysql.inc says

 * Note that if you need to know how many results were returned, you should do
 * a SELECT COUNT(*) on the temporary table afterwards. db_num_rows() and
 * db_affected_rows() do not give consistent result across different database
 * types in this case.

This is untrue. It looks like someone copied the docs for db_query_temporary() and forgot to delete this paragraph, as db_query_range() does not use temporary tables; for MySQL it simply adds a LIMIT clause. This patch removes this comment.