[web] Cut extraneous/unused styles from thread-settings-privacy-tab.css

Summary:
Ripping out styles that aren't going to be relevant once ThreadSettingsPrivacyTab is re-styled.

Depends on D4043

Test Plan:
Before:

After:

Everything continues to work as expected, doesn't really matter how it looks at this stage since it's going to be re-styled in the next diffs.

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4044