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.

PR-URL: #10260
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

lib/fs.js

@@ -1961,7 +1961,6 @@ ReadStream.prototype._read = function(n) {
 
  if (!pool || pool.length - pool.used < kMinPoolSpace) {
  // discard the old pool.
- pool = null;
  allocNewPool(this._readableState.highWaterMark);
  }