[etwist] use more specific REQUEST_ENTITY_TOO_LARGE instead of BAD_REQUEST When a request body exceeds the configured limit, return 413 to the client instead of a generic 400.

[entities, view] delete dead code The auto_unwrap_bw_compat metaclass calls unwrap_adapter_compat, which was removed in changeset 697a8181ba30 "remove 3.9 bw compat". So this can't possibly work anymore, meaning we can get rid of it.