Disable sync checkpoint system that blocks IBD

The sync checkpoint (hashSyncCheckpoint) was persisted in LevelDB pointing
to block 2,186,940. On startup it was loaded from DB, overriding any code
change to the initial value. CheckSync then rejected every block below
that height during IBD since they weren't in mapBlockIndex yet.

Three-pronged fix:
- CheckSync now always returns true (master key disabled, no new sync
  checkpoints will ever be broadcast)
- AcceptBlock no longer calls sync checkpoint enforcement
- LoadBlockIndex resets sync checkpoint to genesis if stored hash is
  not in the block index (prevents assert crash)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/checkpoints.cpp

@@ -218,30 +218,10 @@ namespace Checkpoints
     }
 
     // Check against synchronized checkpoint
+    // Disabled: master key removed in V5, no new sync checkpoints possible.
+    // Always returns true to prevent stale DB-persisted checkpoints from blocking IBD.
     bool CheckSync(const uint256& hashBlock, const CBlockIndex* pindexPrev)
     {
-        if (fTestNet) return true; // Testnet has no checkpoints
-        int nHeight = pindexPrev->nHeight + 1;
-
-        LOCK(cs_hashSyncCheckpoint);
-        // sync-checkpoint should always be accepted block
-        assert(mapBlockIndex.count(hashSyncCheckpoint));
-        const CBlockIndex* pindexSync = mapBlockIndex[hashSyncCheckpoint];
-
-        if (nHeight > pindexSync->nHeight)
-        {
-            // trace back to same height as sync-checkpoint
-            const CBlockIndex* pindex = pindexPrev;
-            while (pindex->nHeight > pindexSync->nHeight)
-                if (!(pindex = pindex->pprev))
-                    return error("CheckSync: pprev null - block index structure failure");
-            if (pindex->nHeight < pindexSync->nHeight || pindex->GetBlockHash() != hashSyncCheckpoint)
-                return false; // only descendant of sync-checkpoint can pass check
-        }
-        if (nHeight == pindexSync->nHeight && hashBlock != hashSyncCheckpoint)
-            return false; // same height with sync-checkpoint
-        if (nHeight < pindexSync->nHeight && !mapBlockIndex.count(hashBlock))
-            return false; // lower height than sync-checkpoint
         return true;
     }
 
@@ -361,8 +341,8 @@ namespace Checkpoints
     bool IsMatureSyncCheckpoint()
     {
         LOCK(cs_hashSyncCheckpoint);
-        // sync-checkpoint should always be accepted block
-        assert(mapBlockIndex.count(hashSyncCheckpoint));
+        if (!mapBlockIndex.count(hashSyncCheckpoint))
+            return true; // no valid sync checkpoint, treat as mature
         const CBlockIndex* pindexSync = mapBlockIndex[hashSyncCheckpoint];
         return (nBestHeight >= pindexSync->nHeight + nCoinbaseMaturity ||
                 pindexSync->GetBlockTime() + nStakeMinAge < GetAdjustedTime());