[services] Tunnelbroker - Remove cryptopp usage from the C++ codebase

Summary:
This diff removes deprecated C++ code related to the use of cryptopp library.
In D5947 we are switched to using the Rust function for the signed string verification and we don't need the cryptopp and related C++ code base anymore.

Removing cryptopp from the dependencies is in the following D5949.

Linear task: ENG-2492

Test Plan: The Tunnelbroker server application is successfully built.

Reviewers: jon, bartek, tomek

Reviewed By: jon, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D5948