Remove assumeutxo / UTXO-snapshot feature

Removes the dumptxoutset RPC, -loadutxosnapshot / -loadutxosnapshotunsafe,
the CCoinsViewDB Dump/LoadSnapshot machinery + CUTXOSnapshotHeader, the
AssumeutxoData chainparams anchor, the LoadSnapshotChainstate activation +
reorg-below-H guard, the persisted assumeutxo-height flag, and the gtest.

Rationale: it duplicated the existing bootstrap (same skip-the-genesis-grind
fast-sync, no speed advantage), its only real edge was a trust model we don't
need for this chain, and it was inert anyway (no published snapshot hash in
chainparams). The -loadutxosnapshot load path adopted an external UTXO set and
bypassed genesis validation, so removing it also drops that attack surface.
Builds clean (no dangling references); the kept IBD speedups (RandomX
pre-verify, adaptive dbcache, tlsmanager) are untouched.

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

src/chainparams.h

@@ -69,17 +69,6 @@ public:
         double fTransactionsPerDay;
     };
 
-    /** Trusted UTXO-snapshot (assumeutxo-style) anchor. When `hash` is set, a node loading a
-     *  snapshot via -loadutxosnapshot must produce exactly this content hash at this height,
-     *  otherwise the snapshot is refused. Null hash = not configured (loading requires the
-     *  explicit -loadutxosnapshotunsafe override, e.g. for regtest/testing). Mirrors the
-     *  hardcoded-checkpoint trust model. */
-    struct AssumeutxoData {
-        int height;
-        uint256 hash;
-        bool IsNull() const { return hash.IsNull(); }
-    };
-
     enum Bech32Type {
         SAPLING_PAYMENT_ADDRESS,
         SAPLING_FULL_VIEWING_KEY,
@@ -116,7 +105,6 @@ public:
     const std::string& Bech32HRP(Bech32Type type) const { return bech32HRPs[type]; }
     const std::vector<uint8_t>& FixedSeeds() const { return vFixedSeeds; }
     const CCheckpointData& Checkpoints() const { return checkpointData; }
-    const AssumeutxoData& Assumeutxo() const { return assumeutxoData; }
     /** Return the founder's reward address and script for a given block height */
     std::string GetFoundersRewardAddressAtHeight(int height) const;
     CScript GetFoundersRewardScriptAtHeight(int height) const;
@@ -156,7 +144,6 @@ protected:
     bool fMineBlocksOnDemand = false;
     bool fTestnetToBeDeprecatedFieldRPC = false;
     CCheckpointData checkpointData;
-    AssumeutxoData assumeutxoData; // null by default; set per-network in chainparams.cpp once a snapshot hash is published
     std::vector<std::string> vFoundersRewardAddress;
 };