protoveneer: remove

Change-Id: I9bc97e6fcaef6fa0bc20ae1105493c81017ffe3a
Reviewed-on: https://go-review.googlesource.com/c/exp/+/554718
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>

Author: jba