[lib] Remove last_[navigated_to/pruned] from ClientDBThreadMessageInfo

Summary:
Last remnants of messageStore.threads[threadID].[lastPruned/lastNavigatedTo].

Depends on D9518

Test Plan: Flow, this code is dead anyways.

Reviewers: ashoat, tomek, ginsu, rohan

Reviewed By: ashoat

Subscribers: wyilio

Differential Revision: https://phab.comm.dev/D9519