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

Summary: These styles were either moved to relevant ThreadSettings[General/Privacy/Delete]Tab component or weren't used to begin with.

Test Plan:
Did a quick scan and things look as expected.

Searched for selector in ThreadSettingsModal before deleting from thread-settings-modal.css

Reviewers: palys-swm, def-au1t

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3749