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

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 1a3558341860357c2400e37773e5076bb3a51628

•	cmd/go: do context propagation for tracing downloads
	CL 248327, commit c0cf190d226cc3defb71d17c01d0b45bf49a8a85

•	cmd/go/internal: remove some users of par.Work
	CL 248326, commit f30044a03bc7cf107dbec03c02fb6d0072878252

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>