[lib] Remove support for LegacyRawThreadInfo in getRelativeMemberInfos

Summary:
Gets things to a much more readable place again.

Based this on the "before" code from D10235 (https://phab.comm.dev/file/data/m6qxcyyopjoqtkp4vuls/PHID-FILE-uwkm4w7aiiwdont5iviy/lib_selectors_user-selectors.js)

Depends on D10806

Test Plan: flow

Reviewers: ashoat, ginsu, tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D10807