Remove final from private functions for PHP 8 compatibility
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
Differential Revision: https://secure.phabricator.com/D21496