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

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

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

Updates #24929
Updates #37203

Change-Id: Ie3bf37894ab68b5b129eff54637893c7a129da03
Reviewed-on: https://go-review.googlesource.com/c/go/+/219540
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>