refactor: Simplify extra_txn to be a vec of CTransactionRef instead of a vec of pair<Wtxid, CTransactionRef>

All `CTransactionRef` have `.GetWitnessHash()` that returns a cached `const Wtxid` (since fac1223a56),
so we don't need to pass transaction refs around with their IDs as they're easy to get from a ref.

src/net_processing.cpp

@@ -1006,7 +1006,7 @@ private:
     /** Orphan/conflicted/etc transactions that are kept for compact block reconstruction.
      *  The last -blockreconstructionextratxn/DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN of
      *  these are kept in a ring buffer */
-    std::vector<std::pair<Wtxid, CTransactionRef>> vExtraTxnForCompact GUARDED_BY(g_msgproc_mutex);
+    std::vector<CTransactionRef> vExtraTxnForCompact GUARDED_BY(g_msgproc_mutex);
     /** Offset into vExtraTxnForCompact to insert the next tx */
     size_t vExtraTxnForCompactIt GUARDED_BY(g_msgproc_mutex) = 0;
 
@@ -1802,7 +1802,7 @@ void PeerManagerImpl::AddToCompactExtraTransactions(const CTransactionRef& tx)
         return;
     if (!vExtraTxnForCompact.size())
         vExtraTxnForCompact.resize(m_opts.max_extra_txs);
-    vExtraTxnForCompact[vExtraTxnForCompactIt] = std::make_pair(tx->GetWitnessHash(), tx);
+    vExtraTxnForCompact[vExtraTxnForCompactIt] = tx;
     vExtraTxnForCompactIt = (vExtraTxnForCompactIt + 1) % m_opts.max_extra_txs;
 }