Revert "os: employ sendfile(2) for file-to-file copying on Linux when needed"

This reverts CL 603295.

Reason for revert: can cause child exit_group to hang.

This is not a clean revert. CL 603098 did a major refactoring of the
tests. That refactor is kept, just the sendfile-specific tests are
dropped from the linux tests.

Fixes #71375.

Change-Id: Ic4d6535759667c69a44bd9281bbb33d5b559f591
Reviewed-on: https://go-review.googlesource.com/c/go/+/644895


Auto-Submit: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Andy Pan <panjf2000@gmail.com>