encoding: remove unused benchV1 flag
encoding: remove unused benchV1 flag

Change-Id: Ib86ab31ffa38f85cbb218c114f0e6697e3df61d8
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221018
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go