Remove committee-user relation

@emanuelschuetze said in https://github.com/OpenSlides/openslides-client/pull/2210#pullrequestreview-1393462131 that the number of assigned accounts for committee managers could be removed. Since this is AFAIK the only use of the relation committee/user-ids, we could remove the relation altogether. This relation (specifically, the calculated fields handler for it) was the root cause for https://github.com/OpenSlides/openslides-backend/issues/1840. We could potentially prevent such errors in the future by removing this relation, maybe together with the meeting-user relation so we no longer have any calculated fields.