[lib] First pass at removing LegacyRelativeMemberInfo from lib

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

Remove LegacyRelativeMemberInfo from lib where straightforward. Will come back and remove most remaining instances after going through web and native and refactoring some components that consume LegacyRelativeMemberInfo and call functions in lib with them.

Depends on D10726

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10727