[web] Remove unused CSS selectors from thread-settings-modal.css

Summary: Remove CSS selectors that are obviously unused.

Test Plan: IDE would give me a red squiggle in JSX if selector was missing.

Reviewers: palys-swm, def-au1t, ashoat

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3623