We only need the memory barrier from these stores,
and we only store nil over nil or over a static function value.
The write barrier is unnecessary.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.0µs ± 0%  -0.43%  (p=0.032 n=5+5)
SetReadDeadline-6      205ns ± 1%   205ns ± 1%    ~     (p=0.683 n=5+5)

Update #25729

Change-Id: I66c097a1db7188697ddfc381f31acec053dfed2c
Reviewed-on: https://go-review.googlesource.com/c/146345
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>