[terraform] Hide prod Tunnelbroker gRPC endpoint from the Internet

Summary:
Addresses a part of ENG-9129.

This stops exposing publicly the Tunnelbroker gRPC endpoint.

Test Plan: terraform plan for staging contains no changes, for prod removes the resource.

Reviewers: will, kamil

Reviewed By: will, kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13245