Outdated warning regarding upload all files as private and use_s3_for_public in settings form

Problem/Motivation

Currently the settings form the following warning appears:

Important:If you enable this feature you can't enable 'use_s3_for_public'.

This is a holdover warning from before #3202394: Allow private upload of public files..

Steps to reproduce

Visit settings page

Proposed resolution

Remove warning from settings page.

Remaining tasks

Remove warning text from settings page.

User interface changes

Warning text removed from settings page.

API changes

None

Data model changes

None