Remove final from private functions for PHP 8 compatibility

Summary:
This combination does not make sense and PHP 8 errors with:

Private methods cannot be final as they are never overridden by other classes

Thus remove the redundant final from all such functions.

Test Plan: Used to create this revision with PHP 8 on macOS

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D21496