Remove unused BDP algorithms and other congestion control algs

In Proposal #324 (closed), we specified 3 different congestion control algorithms, and 3 different BDP estimation algorithms.

Right now, Shadow is indicating that TOR_VEGAS congestion control, and CWND_RTT BDP estimation are the best performers, but we will want to continue to evaluate BDP estimation options as Shadow simulation improves. Right now, we are not confident that Shadow is accurately loading Guard relays, and this could be reducing the reliability of other BDP estimators.

This likely means that we should not remove the other algorithms until 0.4.7-stable at the earliest, and more likely not until 0.4.8.

If for some reason the BDP estimators show up on profiles, we can #ifdef them out in 0.4.7. But they only run once every 31 cells.