remove third-party/

Summary:
Remove the third-party/ directory and no longer rely on copies of dependencies
in the repo. When needed, fetch dependencies normally. This can usually be
accomplished via `pip install -r requirements.txt` or `pip install -r
requirements-dev.txt`.

Even though `peru.yaml` is gone, `.peru/` is still in `.gitignore`, because the
tests check for untracked files and will detect `.peru/` as an error.

Test Plan: Successfully ran tests locally and via Travis CI.

Reviewers: jacko

Differential Revision: https://phabricator.buildinspace.com/D145