cmd/go: delete flaky TestQEMUUserMode

If QEMU user-mode is actually a supported configuration, then per
http://golang.org/wiki/PortingPolicy it needs to have a builder
running tests for all packages, not just a simple “hello world”
program.

Updates #1508
Updates #13024
Fixes #35457

Change-Id: Ib6122b06ad1d265550a0e92131506266495893cc
Reviewed-on: https://go-review.googlesource.com/c/go/+/206137


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>