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.

cpp-samples/pubsub-open-telemetry/vcpkg.json

Line 12 in f214349

"monitoring"

We can remove the workaround when we

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