[lib] Delete old replies count logic

Summary:
In the next diff we're introducing a more unified approach.

https://linear.app/comm/issue/ENG-8930/unify-the-approach-to-replies-count-and-unread-status-updates

Depends on D13057

Test Plan: Flow.

Reviewers: kamil, inka

Reviewed By: inka

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D13058