Remove the field "Use the Unsplash API to fetch images from Unsplash"

Problem/Motivation

The Unsplash plug-in (src/Plugin/media/Source/Unsplash.php
) defined a field named "Use the Unsplash API to fetch images from Unsplash", which doesn't do anything. It doesn't make a lot of sense to keep this field around.

Proposed resolution

- remove the field from the code
- update the documentation

Remaining tasks

- everything

User interface changes

- Field will disappear

API changes

- none

Data model changes

- none