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