[lib] Get rid of switchMemberAdminRoleInThread

Summary:
Working on enabling the unused-promise Flow lint (ENG-5907), and this got flagged because of the dispatchActionPromise usage.

I looked but I couldn't see any usages of this function in the codebase. I think the last usage was removed in D8155.

Depends on D10158

Test Plan: Flow

Reviewers: rohan

Reviewed By: rohan

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10159