test: fix test-trace-events-dns

Test is using `common.fileExists()` which has been removed. There is no
need to check that the file exists as the attempt to read the file in
the next line will fail if the file does not exist. Remove existence
check.

PR-URL: https://github.com/nodejs/node/pull/22674


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>