[lib] Remove simple usages of LegacyRawThreadInfo

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

Following some of the ThreadInfo and ResolvedThreadInfo refactoring, these were some very simple usages we can remove. Will work on getRelativeMemberInfos next.

Depends on D10721

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10724