hygiene: Phase 3 — dead-code excision (~7,100 lines)

Third phase of the code-hygiene remediation: remove provably-dead code.
Each in-file deletion was adversarially verified (a per-finding investigator
plus a skeptic that greps the tree to refute deletability); the sanity grep
confirms zero dangling references, and the tree builds and self-mines clean.

Removed wholesale:
- src/cc/dapps/ (9 files, ~6,939 lines): a git-tracked but never-built Komodo
  DEX / z-migration tool (hushdex.c, zmigrate.c, cJSON.c, ...). Nothing in
  Makefile.am/configure.ac references it.

In-file dead code:
- pow.cpp: the `#ifdef original_algo` oldRT_CST_RST function (the macro is
  never defined in source or build flags) and two `if ( 0 )` debug blocks.
- walletdb.cpp: the "orphaned staking transaction" cleanup path (deadTxns) —
  DragonX is RandomX PoW with no staking, so the guard is always false and the
  block never runs; also drop the now-unused static/extern decls.
- cc/eval.h: the ProcessCC / Eval::ImportCoin / ImportPayout / DisputePayout
  declarations that have no definition anywhere in the tree.
- rpc/crosschain.cpp: the crosschainproof stub RPC (returned {} unconditionally)
  and its registrations in rpc/server.cpp, rpc/server.h, rpc/client.cpp.
- coins.cpp: the commented-out `//TODO: delete` Sprout PushAnchor template.
- wallet.cpp: the permanently-zero KMD `interest2` term in CreateTransaction.
- rpc/net.cpp: hush_longestchain's always-zero `n` var (num > (n>>1) => num>0)
  and its `if ( 0 )` debug branch.
- hush_nSPV.h / hush_nSPV_fullnode.h: three `if ( 0 && ... )` dead debug branches.
- net.cpp, crypter.h, saplingconsolidation.cpp (dup set_error_code),
  shieldcoinbase.cpp (`donation < 0` on a uint8_t), cclib.cpp (unused
  FAUCET2SIZE): single-line dead-code fixes.

Deliberately NOT touched (verification refuted the audit's "dead on DragonX"
premise): the ~2,250-line HUSH3 checkpoint block, the miner notary/timelock
paths, and hush_gateway.h — all reachable at runtime via -ac_name / -ac_*
args (the inherited Komodo assetchain model), and hush_gateway's
hush_opretvalidate is called from ConnectBlock (consensus). Dropping those
requires a deliberate decision to remove HAC/HUSH3 mode, tracked separately.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/wallet/wallet.cpp

@@ -4359,7 +4359,7 @@ bool CWallet::FundTransaction(CMutableTransaction& tx, CAmount &nFeeRet, int& nC
 bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend, CWalletTx& wtxNew, CReserveKey& reservekey, CAmount& nFeeRet,
                                 int& nChangePosRet, std::string& strFailReason, const CCoinControl* coinControl, bool sign)
 {
-    uint64_t interest2 = 0; CAmount nValue = 0; unsigned int nSubtractFeeFromAmount = 0;
+    CAmount nValue = 0; unsigned int nSubtractFeeFromAmount = 0;
     BOOST_FOREACH (const CRecipient& recipient, vecSend)
     {
         if (nValue < 0 || recipient.nAmount < 0)
@@ -4480,7 +4480,6 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend, CWalletTx& wt
                 CAmount nValueIn = 0;
                 bool fOnlyCoinbaseCoins = false;
                 bool fNeedCoinbaseCoins = false;
-                interest2 = 0;
                 if (!SelectCoins(nTotalValue, setCoins, nValueIn, fOnlyCoinbaseCoins, fNeedCoinbaseCoins, coinControl))
                 {
                     if (fOnlyCoinbaseCoins && Params().GetConsensus().fCoinbaseMustBeProtected) {
@@ -4506,8 +4505,7 @@ bool CWallet::CreateTransaction(const vector<CRecipient>& vecSend, CWalletTx& wt
                     dPriority += (double)nCredit * age;
                 }
 
-                CAmount nChange = (nValueIn - nValue + interest2);
-//fprintf(stderr,"wallet change %.8f (%.8f - %.8f) interest2 %.8f total %.8f\n",(double)nChange/COIN,(double)nValueIn/COIN,(double)nValue/COIN,(double)interest2/COIN,(double)nTotalValue/COIN);
+                CAmount nChange = (nValueIn - nValue);
                 if (nSubtractFeeFromAmount == 0)
                     nChange -= nFeeRet;