[lib] Simplify threadMemberHasPermission()
Summary:
This function is only consumed by appendUserInfo which is only consumed in usePotentialMemberItems. This hook is only used on client, so there's no need to support Server* or Legacy* types.
We can also skip .minimallyEncoded branch since all clients work exclusively with MinimallyEncoded* types at this point.
Depends on D11740
Test Plan: NA, flow and read through callsites.
Reviewers: ginsu, tomek, varun, will
Reviewed By: tomek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D11741