fs: remove needless assignment of null

This line `pool = null;` isn't needed and has
been around since the first iteration of streams.
I can't find a good reason for it to exist, it's
not more readable, nor does it seem to trick the
compiler into any optimizations.

lib/fs.js

@@ -1794,7 +1794,6 @@ ReadStream.prototype._read = function(n) {
 
   if (!pool || pool.length - pool.used < kMinPoolSpace) {
     // discard the old pool.
-    pool = null;
     allocNewPool(this._readableState.highWaterMark);
   }