[lib] Remove LegacyThreadInfo where simple in thread-utils

Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedrawthreadinfo-to-rawthreadinfo

Will come back and remove more involved usages.

Depends on D10677

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D10678