all: drop old +build lines

Running 'go fix' on the cmd+std packages handled much of this change.

Also update code generators to use only the new go:build lines,
not the old +build ones.

For #41184.
For #60268.

Change-Id: If35532abe3012e7357b02c79d5992ff5ac37ca23
Cq-Include-Trybots: luci.golang.try:gotip-linux-386-longtest,gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/536237


Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>