[lib] remove updateThreadStoreThreadInfos util function

Summary:
With migration 77, I had a huge misunderstanding about the new migration approach. I thought for this migration we needed to create and directly return a new thread store state to redux using .processStoreOperations(). However, we actually should only just return the operations and just return state as is. The threadStore will be read by SQLiteDataHandler which populates redux with the migrated data from the ops.

Anyways, this util function is unused and can be removed

Depends on D12498

Test Plan: flow + at the end of the stack confirmed that migration 78 works as expected

Reviewers: ashoat, kamil

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12499