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.h

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