[lib] remove newMemberIDs from DMThreadSettingsChanges

Summary:
ENG-8979;

For adding new members there is separate spec, ADD_MEMBERS and ADD_VIEWER_TO_THREAD_MEMBERS.

Depends on D13192

Test Plan: Flow

Reviewers: tomek, inka, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13195