test: remove unused --expose-native-as V8 flag

test-preload.js was using a V8 flag (`--expose-native-as`) that made
an V8 internally used object available. As this test does not use this
object, this commit removes the usage of this flag.

In some distant past, this internally used object may have had some
external use, but currently is essentially an empty object.

In the near future, the V8 internal infrastructure (JS Natives)
producing the object exposed by `--expose-native-as` will be phased out.
For more details, visit:
https://bugs.chromium.org/p/v8/issues/detail?id=7624

PR-URL: #25275
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>

Author: peterwmwong

test/parallel/test-preload.js

@@ -135,7 +135,7 @@ childProcess.exec(
 // https://github.com/nodejs/node/issues/1691
 process.chdir(fixtures.fixturesDir);
 childProcess.exec(
-  `"${nodeBinary}" --expose_natives_as=v8natives --require ` +
+  `"${nodeBinary}" --require ` +
      `"${fixtures.path('cluster-preload.js')}" cluster-preload-test.js`,
   function(err, stdout, stderr) {
     assert.ifError(err);