[lib] Delete remove members spec

Summary:
We don't support removing members and maintaining this code doesn't make sense.

https://linear.app/comm/issue/ENG-9823/fix-sending-notifs-about-leaving-a-thick-thread

Depends on D13843

Test Plan: Flow

Reviewers: kamil, angelika

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13845