Simplify type of Cardano.Wallet.constructTransaction.

The function `Cardano.Wallet.constructTransaction` is only ever called
with a `Left PreSelection`, and never called with a `Right Selection`.

Therefore, we can simplify the selection argument type from
`Either PreSelection Selection` to just `PreSelection`.

lib/wallet/api/http/Cardano/Wallet/Api/Http/Shelley/Server.hs

@@ -2439,7 +2439,7 @@ constructTransaction
  , extraCoinSink = fromMaybe (Coin 0) deposit
  }
  unbalancedTx <- liftHandler $
- W.constructTransaction @_ @s @k @n wrk wid era txCtx' (Left preSel)
+ W.constructTransaction @_ @s @k @n wrk wid era txCtx' preSel
 
  let balancedPostData = ApiBalanceTransactionPostData
  { transaction = ApiT unbalancedTx

lib/wallet/src/Cardano/Wallet.hs

@@ -2532,15 +2532,15 @@ constructTransaction
  -> WalletId
  -> Cardano.AnyCardanoEra
  -> TransactionCtx
- -> Either PreSelection (SelectionOf TxOut)
+ -> PreSelection
  -> ExceptT ErrConstructTx IO SealedTx
-constructTransaction ctx wid era txCtx sel = db & \DBLayer{..} -> do
+constructTransaction ctx wid era txCtx preSel = db & \DBLayer{..} -> do
  (_, xpub, _) <- withExceptT ErrConstructTxReadRewardAccount $
  readRewardAccount @ctx @s @k @n ctx wid
  mapExceptT atomically $ do
  pp <- liftIO $ currentProtocolParameters nl
  withExceptT ErrConstructTxBody $ ExceptT $ pure $
- mkUnsignedTransaction tl era xpub pp txCtx sel
+ mkUnsignedTransaction tl era xpub pp txCtx (Left preSel)
  where
  db = ctx ^. dbLayer @IO @s @k
  tl = ctx ^. transactionLayer @k @'CredFromKeyK