message: validation: Move PruneOneBlockFile to BlockManager
  [META] This is a pure refactor commit.
  Move PruneBlockFile to BlockManager because:
  1. PruneOneBlockFile only acts on BlockManager
  2. Eliminates the need for callers (FindFilesToPrune{,Manual}) to have a
  reference to the larger ChainstateManager, just a reference to
  BlockManager is enough. See following commits.
diff: @@ -377,6 +377,9 @@ public:
  /** Create a new block index entry for a given block hash */
  CBlockIndex* InsertBlockIndex(const uint256& hash) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
  + //! Mark one block file as pruned (modify associated database entries)
  + void PruneOneBlockFile(const int fileNumber) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
  +
  /**
  * If a block header hasn't already been seen, call CheckBlockHeader on it, ensure
  * that it doesn't descend from an invalid block, and then add it to m_block_index.
  @@ -873,9 +876,6 @@ public:
  */
  bool ProcessNewBlockHeaders(int32_t& nPoSTemperature, const uint256& lastAcceptedHeader, const std::vector<CBlockHeader>& block, BlockValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex = nullptr) LOCKS_EXCLUDED(cs_main);
  - //! Mark one block file as pruned (modify associated database entries)
  - void PruneOneBlockFile(const int fileNumber) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
  -
  //! Load the block tree and coins database from disk, initializing state if we're running with -reindex
  bool LoadBlockIndex(const CChainParams& chainparams) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
  