os/exec: remove protection against simultaneous Wait/Write

os/exec: remove protection against simultaneous Wait/Write

CL 31148 added code to protect again simultaneous calls to Close and
Wait when using the standard input pipe, to fix the race condition
described in issue #9307. That issue is a special case of the race
between Close and Write described by issue #7970. Since issue #7970
was not fixed, CL 31148 fixed the problem specific to os/exec.

Since then, issue #7970 has been fixed, so the specific fix in os/exec
is no longer necessary. Remove it, effectively reverting CL 31148 and
followup CL 33298.

Updates #7970
Updates #9307
Updates #17647

Change-Id: Ic0b62569cb0aba44b32153cf5f9632bd1f1b411a
Reviewed-on: https://go-review.googlesource.com/65490
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Miguel Bernabeu <miguelbernadi@gmail.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Joe Tsai <joetsai@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>