d8: Remove experimental notice from stderr

[ptomato]

If you invoke d8 with any --harmony flags, it prints a warning to stderr that you are using experimental technologies. Remove this warning from the captured stderr, because it's just noise - if you are running d8 with a --harmony flag via eshost, you probably know what you are doing.

Additionally, non-empty stderr makes test262harness think the test has failed, so it's currently impossible to run test262 tests for a --harmony feature.