[web] [refactor] remove unneeded div in chat-thread-list component

Summary:
remove an extra div we don't need. I hard-coded a color but will remove it in future diffs, along with other unneeded styles. I just didn't want to break existing functionality.

before:

Test Plan: do a side-by-side with production. Make sure the unread pop up doesn't break.

Reviewers: atul, def-au1t, palys-swm

Reviewed By: palys-swm

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

Differential Revision: https://phabricator.ashoat.com/D3405