test: remove unused var from stream2 test

`writes` is assigned but never used. Remove it.

(This was missed by the linter in previous versions of ESLint but is
flagged by the current version. Updating the linter is contingent on
this change or some similar remedy landing.)

PR-URL: #7596
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

test/parallel/test-stream2-readable-legacy-drain.js

@@ -19,10 +19,8 @@ r.on('end', function() {
 
 var w = new Stream();
 w.writable = true;
-var writes = 0;
 var buffered = 0;
 w.write = function(c) {
-  writes += c.length;
   buffered += c.length;
   process.nextTick(drain);
   return false;