rpc: clarify ALREADY_IN_CHAIN rpc errors

When using `sendrawtransaction` the ALREADY_IN_CHAIN error help string
may be confusing.

Rename TransactionError::ALREADY_IN_CHAIN to
TransactionError::ALREADY_IN_UTXO_SET and update the rpc help string.

Remove backwards compatibility alias as no longer required.

src/node/transaction.cpp

@@ -55,7 +55,7 @@ TransactionError BroadcastTransaction(NodeContext& node, const CTransactionRef t
             const Coin& existingCoin = view.AccessCoin(COutPoint(txid, o));
             // IsSpent doesn't mean the coin is spent, it means the output doesn't exist.
             // So if the output does exist, then this transaction exists in the chain.
-            if (!existingCoin.IsSpent()) return TransactionError::ALREADY_IN_CHAIN;
+            if (!existingCoin.IsSpent()) return TransactionError::ALREADY_IN_UTXO_SET;
         }
 
         if (auto mempool_tx = node.mempool->get(txid); mempool_tx) {

src/node/types.h

@@ -19,7 +19,7 @@ namespace node {
 enum class TransactionError {
     OK, //!< No error
     MISSING_INPUTS,
-    ALREADY_IN_CHAIN,
+    ALREADY_IN_UTXO_SET,
     MEMPOOL_REJECTED,
     MEMPOOL_ERROR,
     MAX_FEE_EXCEEDED,