validation: remove unused using directives

The following were unused from the node namespace:
- BLOCKFILE_CHUNK_SIZE
- nPruneTarget
- OpenBlockFile
- UNDOFILE_CHUNK_SIZE

src/validation.cpp

@@ -69,7 +69,6 @@ using kernel::ComputeUTXOStats;
 using kernel::LoadMempool;
 
 using fsbridge::FopenFn;
-using node::BLOCKFILE_CHUNK_SIZE;
 using node::BlockManager;
 using node::BlockMap;
 using node::CBlockIndexHeightOnlyComparator;
@@ -77,11 +76,8 @@ using node::CBlockIndexWorkComparator;
 using node::fImporting;
 using node::fPruneMode;
 using node::fReindex;
-using node::nPruneTarget;
-using node::OpenBlockFile;
 using node::ReadBlockFromDisk;
 using node::SnapshotMetadata;
-using node::UNDOFILE_CHUNK_SIZE;
 using node::UndoReadFromDisk;
 using node::UnlinkPrunedFiles;