Remove some unused fields from yy_extra_type

Summary: These fields are no longer used after D1319. Removed them should save some precious bytes.

Test Plan: Ran arc unit -- src/parser/xhpast/__tests__/PHPASTParserTestCase.php.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13973