cmd/dist: remove pkg/$GOOS_$GOARCH and pkg/obj at exit

pkg/obj will be empty, so just remove it.
pkg/$GOOS_$GOARCH will be empty unless the user has specified
GODEBUG=installgoroot=all, so check if it's empty, and if so, delete
it.

Also remove xreaddirfiles, which is unused.

Also remove the copy of pkg/$GOOS_$GOARCH in the cmd/go test
TestNewReleaseRebuildsStalePackagesInGOPATH. The directory is empty so
copying it has no effect.

For #47257

Change-Id: Ief90b882d157bd16078cd5d2b83a915bfc831f9a
Reviewed-on: https://go-review.googlesource.com/c/go/+/453496


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>