[web] Move onDelete up from ThreadSettingsModal to ConnectedThreadSettingsModal

Summary:
Move onDelete out from inner class component to outer "Connected" functional component.

As part of the work to refactor ThreadSettingsModal... specifically to turn it into a functional component

Depends on D3783

Test Plan:

1. Create delete-able thread
2. Navigate to ThreadSettingsModal and go to "Delete" tab
3. Try to delete thread and ensure that it works as expected

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3794