[web] Remove unused/unnecessary selectors from thread-settings-delete-tab.css

Summary:

1. Remove the textarea selector which isn't used
2. Remove the "line" separating the notice from the instructions to delete

Depends on 4016

Test Plan:
Before:

After:

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4017