Remove protobuf_src from example

[Jake-Shadle]

Noticed an example still pulled this in, unnecessarily slowing down CI runs.

[quilkin-bot]

Build Succeeded 🥳

Build Id: 5ab79758-ac2c-4181-88f1-512a0d0ed252

The following development images have been built, and will exist for the next 30 days:

• us-docker.pkg.dev/quilkin/ci/quilkin:0.8.0-dev-cb29172

To build this version:

git fetch git@github.com:googleforgames/quilkin.git pull/898/head:pr_898 && git checkout pr_898
cargo build