Remove references to deprecated protobuf module

[acsecureworks]

migrated everything from proto2 to proto3
all tests pass when run locally

breaking changes:

1. protobuf default removed
2. all protobuf v2 group element replaced with submessages
3. protobuf v2 extension lines removed

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[acsecureworks]

signed

[trakhimenok]

Any chance go get it merged?