[keyserver] Remove single instance of LegacyThreadInfo from keyserver

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

This is safe because there's only one preparePushNotif callsite and we're guaranteed to get minimally encoded ThreadInfos back from threadInfoFromRawThreadInfo(...):

Depends on D10678

Test Plan: flow, this should be a noop

Reviewers: ashoat, ginsu, tomek

Reviewed By: ginsu

Differential Revision: https://phab.comm.dev/D10713