Search backend: remove unused exported functions
Created by: camdencheek
I started by just removing IsStreamingCompatible since I recently removed the last references to it, but noticed there were a significant number of dead exported functions in this package.
I'm happy to resurrect any if they are going to be used, but I think as part of our backend cleanup, it will be helpful to have packages that are as lightweight as possible.
Test plan
Just removing dead code, so any issues will be caught statically.