Revert "cmd/go: support cgo files in overlays"
This reverts CL 267197.
Reason for revert: tests failing on windows-amd64-longtest builder (https://build.golang.org/log/
83afde1aac3ea49debddb8996d089a741c3b1cf1).
Change-Id: I0d7c706144e34b1715316ca2ee9662c901b7f8f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/267357
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>