Cleanup vcpkg manifest for pubsub + OTel

[dbolduc]

Our opentelemetry feature should depend on monitoring in our vcpkg port. We should not have to manually depend on it in this example's manifest file.

https://github.com/GoogleCloudPlatform/cpp-samples/blob/f214349286e78b1fa5466efa82196e2bc76fd0d1/pubsub-open-telemetry/vcpkg.json#L12

We can remove the workaround when we

• update our vcpkg port
• update the vcpkg version used in our CI