[lib] Remove assertThreadStoreThreadsAreEqual from thread-reducer

Summary:
We removed the non-ops approaches of updating the threadStore, so there aren't any more usages of assertThreadStoreThreadsAreEqual in the codebase.

Depends on D4194

Test Plan: Searched codebase for "assertThreadStoreThreadsAreEqual" and found no occurences.

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4195