[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