os/exec: recombine goroutinePipes and userPipes

os/exec: recombine goroutinePipes and userPipes

This change undoes CL 401894, because on further consideration
it turns out not to be needed.

This also makes (*Cmd).closeDescriptors a free function, since it does
not actually use the receiver in any way and is not needed to satisfy
any interfaces.

For #50436.

Change-Id: I7915265b0e6398ed5a34fae0c12873ab08a14194
Reviewed-on: https://go-review.googlesource.com/c/go/+/437175
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>