Remove "final" from "private" methods in Phabricator

Summary:
Ref T13588. This has never been meaningful, but a "final private" method is specifically forbidden in PHP8.

Remove meaningless "final" from these methods, per new lint checks.

Test Plan: Ran arc lint --everything to identify affected methods, then ... | xargs -n1 arc lint --apply-patches.

Maniphest Tasks: T13588

Differential Revision: https://secure.phabricator.com/D21540