testing: remove obsolete comment in testing.(*T) docs

[sthagen]

We now only accumulate logs when not using -v. Just drop the sentence
entirely rather than try to describe the current situation.

Updates golang#24929
Updates golang#37203

Change-Id: Ie3bf37894ab68b5b129eff54637893c7a129da03
Reviewed-on: https://go-review.googlesource.com/c/go/+/219540
Run-TryBot: Ian Lance Taylor [email protected]
TryBot-Result: Gobot Gobot [email protected]
Reviewed-by: Brad Fitzpatrick [email protected]

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