Drop `upload-button` feature #2014

The feature is currently broken. It was initially added because we hide this bar via CSS:

<img width="403" alt="" src="https://user-images.githubusercontent.com/1402241/57218510-108a1b80-7028-11e9-8fb2-a07066175553.png">

However now we have two reasons why this is not useful:

• upload-button now also takes care of hiding the bar; i.e. the feature can be disabled altogether and the bar reappears
• we now also offer a CSS field to restore the bar

Additionally:

• We have a related feature that drops the Upload files button from file lists and does not add an alternative. Likewise, people can just upload files via drag-drop or disable the feature.

Therefore we can either:

• 👎 Keep the feature as JS, but not add the alternative button
• 👍 Drop the JS feature and just hide the bar via CSS (who doesn't want it can undo it easily in the custom CSS field)