protoveneer: remove

Change-Id: I9bc97e6fcaef6fa0bc20ae1105493c81017ffe3a
Reviewed-on: https://go-review.googlesource.com/c/exp/+/554718
Run-TryBot: Jonathan Amsterdam <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>

Author: jba