Move blockstorage-related unload to BlockManager::Unload

This is a refactor and safe to do because:
* UnloadBlockIndex calls ChainstateManager::Unload, which calls
  BlockManager::Unload
* Only unit tests call Unload directly

src/node/blockstorage.cpp

@@ -348,6 +348,11 @@ void BlockManager::Unload()
     }
 
     m_block_index.clear();
+
+    vinfoBlockFile.clear();
+    nLastBlockFile = 0;
+    setDirtyBlockIndex.clear();
+    setDirtyFileInfo.clear();
 }
 
 bool BlockManager::WriteBlockIndexDB()

src/validation.cpp

@@ -3980,10 +3980,6 @@ void UnloadBlockIndex(CTxMemPool* mempool, ChainstateManager& chainman)
     chainman.Unload();
     pindexBestHeader = nullptr;
     if (mempool) mempool->clear();
-    vinfoBlockFile.clear();
-    nLastBlockFile = 0;
-    setDirtyBlockIndex.clear();
-    setDirtyFileInfo.clear();
     g_versionbitscache.Clear();
     for (int b = 0; b < VERSIONBITS_NUM_BITS; b++) {
         warningcache[b].clear();