[tests] Adjust MessageHandlerTest.GHIssue16339 slightly.

[rolfbjarne]

We've recently had a few test failures like this:

MonoTests.System.Net.Http.MessageHandlerTest.GHIssue16339
 	[FAIL] GHIssue16339() :   IsSuccessStatusCode #1
   Expected: True
   But was:  False

which doesn't provide much info into what went wrong.

It turns out this particular assert for IsSuccessStatusCode is redundant,
because we assert on the exact StatusCode a bit later, and if that assert
fails, we'll know a bit more.

So just remove this redundant assert for IsSuccessStatusCode.

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