[web] Address remaining code review comments from D3341

Summary:
Rename activeThreadCurrentlyUnread and delete unused variable
Depends on D3341
Depends on D3387

Test Plan: Flow

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac, atul

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3388