[web] [remove] lastActivity from thread

Summary:
talked IRL with ashoat about removing timestamp

follow up diff here: https://phabricator.ashoat.com/D3163

Test Plan: make sure page renders

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3098