[benchmark] Don't pass --sound-null-safety to dart_precompiled_runtime2.
[benchmark] Don't pass --sound-null-safety to dart_precompiled_runtime2.

The autodetection was fixed in bbc8aedada6f9670c21fd142cf7771f55d6c5409,
so the workaround of passing --sound-null-safety to the AOT runtime is
no longer needed.

This change tests that the autodetection does in fact work.

Bug: https://github.com/dart-lang/sdk/issues/42421
Change-Id: Ibbe9665905e817a28964b698e6c437897c9dc6ee
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/153771
Reviewed-by: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk