vendor: revert stray edit from CL 406835

vendor: revert stray edit from CL 406835

In reviewing CL 406835, I missed that one of the edited files was in
src/vendor. This change reverts that file, fixing the failing
moddeps test on the longtest builders.

Change-Id: Id04b45c3379cf6c17b333444eb7be1301ffcb5f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/406895
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>