MINOR: [C++] Remove nonexistent class from io/type_fwd.h (#14254)

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Yibo Cai <yibo.cai@arm.com>

Author: lidavidm

cpp/src/arrow/io/type_fwd.h

@@ -66,10 +66,8 @@ class ReadWriteFileInterface;
 
 class LatencyGenerator;
 
-class BufferReader;
-
-class BufferInputStream;
 class BufferOutputStream;
+class BufferReader;
 class CompressedInputStream;
 class CompressedOutputStream;
 class BufferedInputStream;