cleanup(engine): remove the TProxy abstraction

[bassosimone]

Around one year ago we experimented using the TProxy abstraction for implementing integration testing. Where this experiment had been partially successful in allowing us to write integration testing, we eventually stopped it midway when we removed the Fall 2021 prototype of the websteps experiment. Since then, this code is basically unused. What's more, it is also becoming partially annoying because it introduces an interface between the high-level part of netxlite and the getaddrinfo engine (which I noticed while working on #1516). So, my argument here is that the code that survived from the removal of websteps Fall 2021 is ~50 LoC of unused abstractions and we probably want to remove this unused abstraction. Shall we need something similar in the future, we could introduce it again. Recently, @fortuna mentioned to me that probably shadowsocks is a good way to have a VPN on a probe (e.g., a mobile probe) that moves the data outside the device into a network in which we implement censorship for testing. If we implement such a plan, probably we need something slightly different from what we used to do with TProxy. Hence, I am going to remove TProxy.