[lib] Remove LegacyThreadInfo-related type utilities
Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedrawthreadinfo-to-rawthreadinfo
Get rid of minimallyEncode*/decode* utilities.
Depends on D10718
Test Plan: flow
Reviewers: ashoat, ginsu, tomek
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D10720