validation: Move FindFilesToPrune{,Manual} to BlockManager

[META] No behaviour change is intended in this commit.
[META] This commit should be followed up by removing the comments and
       assertions meant only to show that the change is correct.

Also stop FindFilesToPrune{,Manual} from unnecessary reaching for
::ChainActive() by passing in the necessary information.

src/validation.h

@@ -356,7 +356,15 @@ struct CBlockIndexWorkComparator
  * This data is used mostly in `CChainState` - information about, e.g.,
  * candidate tips is not maintained here.
  */
-class BlockManager {
+class BlockManager
+{
+    friend CChainState;
+
+private:
+    // See definition for documentation
+    void FindFilesToPruneManual(std::set<int>& setFilesToPrune, int nManualPruneHeight, int chain_tip_height);
+    void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfterHeight, int chain_tip_height, bool is_ibd);
+
 public:
     BlockMap m_block_index GUARDED_BY(cs_main);