[web] Delete unused user settings modal

Summary:
After we introduced account settings, we're no longer using user settings modal and it can be safely deleted.

Depends on D4121

Test Plan: Flow. Search the codebase for <UserSettingsModal and user-settings-modal - there are no uses.

Reviewers: def-au1t, atul, benschac

Reviewed By: atul

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4125