[web] Remove unused mostRecentReadThread prop from App

Summary:
In D3341 we realized that it might be a good idea to rename mostRecentReadThread to mostRecentlyReadThread. We've chosen that name in app-switcher because the selector is named in that way. To keep it consistent, I decided to rename also the selector, but it appeared that it was used in other places. The usage in App can be just removed as the value is not used at all.
Depends on D3341

Test Plan: Flow

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3386