Remove /containers/ps from the api router.

This route was deprecated more than two years ago in the linked
commit[1]. It's not referenced anywhere in the documentation and it's
time to stop maintaning it.

[1]: https://github.com/docker/docker/commit/4f9443927e8bc8a724b43afae6cf7a183cd9acd0

Signed-off-by: David Calavera <david.calavera@gmail.com>