validation: Move ~CMainCleanup logic to ~BlockManager

~CMainCleanup:
1. Is vestigial
2. References the g_chainman global (we should minimize g_chainman refs)
3. Only acts on g_chainman.m_blockman
4. Does the same thing as BlockManager::Unload

src/validation.cpp

@@ -5226,20 +5226,6 @@ double GuessVerificationProgress(const ChainTxData& data, const CBlockIndex *pin
     return std::min<double>(pindex->nChainTx / fTxTotal, 1.0);
 }
 
-class CMainCleanup
-{
-public:
-    CMainCleanup() {}
-    ~CMainCleanup() {
-        // block headers
-        BlockMap::iterator it1 = g_chainman.BlockIndex().begin();
-        for (; it1 != g_chainman.BlockIndex().end(); it1++)
-            delete (*it1).second;
-        g_chainman.BlockIndex().clear();
-    }
-};
-static CMainCleanup instance_of_cmaincleanup;
-
 Optional<uint256> ChainstateManager::SnapshotBlockhash() const {
     if (m_active_chainstate != nullptr) {
         // If a snapshot chainstate exists, it will always be our active.

src/validation.h

@@ -416,6 +416,10 @@ public:
         BlockValidationState& state,
         const CChainParams& chainparams,
         CBlockIndex** ppindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+
+    ~BlockManager() {
+        Unload();
+    }
 };
 
 /**