cmd/go: revert 3 CLs affecting par.Work, context propagation, tracing

This reverts the following changes:

•	cmd/go: add tracing for querying and downloading from the proxy
	CL 242786, commit 1a355834

•	cmd/go: do context propagation for tracing downloads
	CL 248327, commit c0cf190d

•	cmd/go/internal: remove some users of par.Work
	CL 248326, commit f30044a0

Reason for revert: broke linux 386 and amd64 longtest builders.

The problem started with CL 248326, but CL 248327 and CL 242786
are reverted as well due to conflicts.

Updates #38714.
Fixes #40861.

Change-Id: I68496b4e5a27e47a42183553c3a645b288edac83
Reviewed-on: https://go-review.googlesource.com/c/go/+/249017


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>