refactor: Drop unused fclose() from BufferedFile

This was only explicitly used in the tests, where it can be replaced by
wrapping the original raw file pointer into a CAutoFile on creation and
then calling CAutoFile::fclose().

Also, it was used in LoadExternalBlockFile(), where it can also be
replaced by the (implicit call to the) CAutoFile destructor after
wrapping the original raw file pointer in a CAutoFile.

src/validation.cpp

@@ -4589,7 +4589,6 @@ void ChainstateManager::LoadExternalBlockFile(
 
     int nLoaded = 0;
     try {
-        // This takes over fileIn and calls fclose() on it in the BufferedFile destructor
         BufferedFile blkdat{fileIn, 2 * MAX_BLOCK_SERIALIZED_SIZE, MAX_BLOCK_SERIALIZED_SIZE + 8, CLIENT_VERSION};
         // nRewind indicates where to resume scanning in case something goes wrong,
         // such as a block fails to deserialize.