[web] Remove no longer relevant styles from thread-settings-general-tab.css

Summary:
There were some arbitrary margin values that were set as part of the old design. Removing the selectors/styles from the stylesheet and corresponding component to clean them up before working on ThreadSettingsGeneralTab layout/styling

Depends on D3930

Test Plan:
ThreadSettingsGeneralTab component continues to look fine:

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3932