fs: remove unused default option in WriteStream

No bufferSize option is used in stream.Writable and fs.WriteStream.

Author: Shigeki Ohtsu

lib/fs.js

@@ -1559,7 +1559,6 @@ function WriteStream(path, options) {
 
   // a little bit bigger buffer and water marks by default
   options = util._extend({
-    bufferSize: 64 * 1024,
     lowWaterMark: 16 * 1024,
     highWaterMark: 64 * 1024
   }, options || {});