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

Summary:
Really should have been part of D3845 when I removed the old tabs. But basically cut the unused styles left over from the removed Tabs component.

Depends on D3928

Test Plan: No occurences of tab_panel in the codebase

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3929