Clear fHavePruned in BlockManager::Unload()

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Code Reviewer Notes

Call graph of relevant functions:

UnloadBlockIndex() <-- Moved from
    calls ChainstateManager::Unload()
        which calls BlockManager::Unload() <-- Moved to

So calling UnloadBlockIndex() would still run this moved code. The code
will also now run when ~BlockManager gets called, which makes sense.

src/node/blockstorage.cpp

@@ -300,6 +300,8 @@ void BlockManager::Unload()
     m_last_blockfile = 0;
     m_dirty_blockindex.clear();
     m_dirty_fileinfo.clear();
+
+    fHavePruned = false;
 }
 
 bool BlockManager::WriteBlockIndexDB()