Stop embedding boost

[BoPeng]

We went through all the trouble to embed the boost C++ library because

1. It was quite troublesome for users to install boost by themselves
2. Some versions of boost did not work

For 1, since conda is now the preferred distribution channel and conda has a boost package, installing boost is now trivial.

For 2, the version that did not work (1.41.0) was really old, and we should not care as long as the recent versions work. In the worst case we can specify a version with conda.