sync: delete dummy builders_test file

The builders should be happy building errgroup instead now.

Change-Id: I99f549032f771e067b5f7780eba1cc6e91bbdfa5
Reviewed-on: https://go-review.googlesource.com/24962
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>