TxId in ShelleyTx is redundant

This type family could be turned into a `newtype` instead of data, since `Tx era` already stores memoized version of `TxId`, which is what `txIdTx` actually extracts [`mkShelleyTx`](https://github.com/IntersectMBO/ouroboros-consensus/blob/62ede713d5e56633e575bc7e18dac2ef5c56771e/ouroboros-consensus-cardano/src/shelley/Ouroboros/Consensus/Shelley/Ledger/Mempool.hs#L189-L190)
https://github.com/IntersectMBO/ouroboros-consensus/blob/62ede713d5e56633e575bc7e18dac2ef5c56771e/ouroboros-consensus-cardano/src/shelley/Ouroboros/Consensus/Shelley/Ledger/Mempool.hs#L115-L116

In other words calling `txIdTx` does not recompute the hash, so there is no reason to store it twice.

Same applies to `ShelleyValidatedTx`

Start in ouroboros-consensus-cardano/src/shelley/Ouroboros/Consensus/Shelley/Ledger/Mempool.hs, especially mkShelleyTx and txIdTx. Inspect ShelleyTx and ShelleyValidatedTx to confirm where TxId is stored and extracted; done means both types no longer store the redundant value while their existing behavior remains intact.

Written by the indexing model from the issue text.