benchmark: remove unused variables

Remove variables that are assigned but never used.

(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/7600


Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>