Remove xhpast_parse methods

Summary: These methods are no longer used and have been replaced with static method calls on the PhutilXHPASTBinary class. Depends on D11612, D11613 and D11614.

Test Plan: grep

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11615