Remove ordered_group workaround because groups now preserve order

Remove gwcelery.tasks.core.ordered_group. Group results now preserve order, due to a patch that we contributed. This was one of the major reasons for the Celery 5 upgrade.

There are two related helper tasks, gwcelery.tasks.core.ordered_group_first and gwcelery.tasks.core.ordered_group_last. These need to be simplified, rewritten, and renamed to gwcelery.tasks.core.first and gwcelery.tasks.core.last, respectively.

Although this is basically a code cleanup task, it will also reduce the Redis database size because the intermediate group results will be stored fewer times.