cmd/go: revert multi-flag GOFLAGS doc example

This partially reverts https://golang.org/cl/135035.

Reason for revert: multiple -ldflags=-foo flags simply override each
other, since that's the logic for per-package flags. The suggested
'GOFLAGS=-ldflags=-s -ldflags=-w' has never worked for 'go build', and
even breaks 'go test' and 'go vet'.

There should be a way to specify -ldflags='-w -s' via GOFLAGS, which is
being tracked in #29096. For now, just remove the incorrect suggestion.

Fixes #29053.

Change-Id: I9203056f7e5191e894bcd16595a92df2fb704ea7
Reviewed-on: https://go-review.googlesource.com/c/152479


Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>