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: https://github.com/nodejs/node/pull/7596


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