feedback kcm: Make it about Plasma

Summary:
At the moment we were merging two different concepts into a same KCM and it was
too confusing.
Simplify making the kcm only about Plasma by removing the kill switch and
making sure the text only affects Plasma components.

Reviewers: #plasma, vkrause, davidedmundson, ngraham

Reviewed By: #plasma, davidedmundson, ngraham

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24733