test: remove superfluous check in pummel/test-timers

If setTimeout is not a function, this test and most other tests would
fail. There is no need to check it.

PR-URL: https://github.com/nodejs/node/pull/34488


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Pranshu Srivastava <rexagod@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>