[keyserver][lib] Remove checks for now-deprecated codeVersions in thread fetching code

Summary:
See the first diff in the stack. We're no longer expecting to see these old codeVersions, so we're simplifying this code.

Depends on D8504

Test Plan: I tested this whole stack together by playing around with the web app and making sure thread mutation endpoints seemed to work correctly

Reviewers: michal, kamil

Reviewed By: kamil

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D8505