Remove the HTML/CSS validation plug-in
Summary:
This relies largely on KHTML (or in some cases on WebKit) and is therefore
no longer functional.
Reviewers: dfaure, stefanocrocco
Reviewed By: dfaure
Subscribers: kde-doc-english
Tags: Documentation
Maniphest Tasks: T11543
Differential Revision: https://phabricator.kde.org/D23901