[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