Once #2316 and #2178 are wrapped up, delete any old code and tests we don't need anymore.

• 2.x posts api #2316
• 2.x Update Timber Object Getter function names in Twig #2178

Potential purge candidates:

• PostGetter & tests
• TermGetter & tests
• Any old $CommentClass props and the like
• QueryIterator & tests
• Post::determine_id() and possibly other methods
• Timber_WP_CLI_Command.php (or move to its own repo)