cmd/go: remove cross-package assembly reference discovery

This removes the special case for finding assembly references to Go
symbols in runtime and runtime/internal/atomic. These are no longer
necessary because we've now marked all symbols in these packages that
must be accessible from assembly in other packages.

Fixes #31230.

Change-Id: I70c90b70e13b922a6669f3d46c53347f98d6fc3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/179863


Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>