Commit 9c9e36b3 pushed these errors down to where the write barriers
are actually emitted, but forgot to remove the original error that was
being pushed down.

Change-Id: I751752a896e78fb9e63d69f88e7fb8d1ff5d344c
Reviewed-on: https://go-review.googlesource.com/10264Reviewed-by: Russ Cox <rsc@golang.org>