Reconsider `anchorFromPoint`

`anchorFromPoint` is currently rendered like so:
https://github.com/input-output-hk/ouroboros-network/blob/d2fdcd0ca03dacec156f49d8932e511625938e7b/ouroboros-network-api/src/Ouroboros/Network/AnchoredFragment.hs#L160-L166

In https://github.com/input-output-hk/ouroboros-network/pull/4660#pullrequestreview-1600810364, we discussed a couple of avenues to remediating this function's partial nature, among them:
- Deprecating and deleting it (since it appears unused)
- Making the argument a `RealPoint`, a structure [defined in `ouroboros-consensus`](https://github.com/input-output-hk/ouroboros-consensus/blob/903ba3c7d2f8d8c015b59f0c664cf75c0f280c25/ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Block/RealPoint.hs#L43-L45) that by construction cannot be genesis

Whether to do either of these things, or to do nothing, is an open question.