Remove `phpast.*` Conduit methods

Summary:
Ref T4592. These were added with the intent of not requiring builds on Windows, but then we got builds on Windows working and they seem to be straightforward. See T4592 for most recent discussion.

Remove these methods because they aren't really practical for anything and increase attack surface area by giving adversaries access to `xhpast`, and generally bloat up the Conduit API. To my knowledge, nothing has ever called them.

(If an install somehow relies on these, they can drop them into `src/extensions/` to expose them again.)

Test Plan: Viewed conduit.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4592

Differential Revision: https://secure.phabricator.com/D8500