[Tunnelbroker] Remove references to cxx

Summary: We no longer use cxx to create a c++ to rust bridge. Just remove.

Test Plan:

cd services/tunnelbroker
cargo run

Reviewers: varun, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7659