devel/kio-extras: Remove the htmlthumbnailer.

Albert Astals Cids reports:

The HTML thumbnailer was incorrectly accessing some content of
remote URLs listed in HTML files. This meant that the owners of the servers
referred in HTML files in your system could have seen in their access logs
your IP address every time the thumbnailer tried to create the thumbnail.

Use the suggested workaround, and remove the htmlthumbnailer.

MFC after: 2018Q4
Security: 1460aa25-e6ab-11e8-a733-e0d55e2a8bf9
Security: CVE-2018-19120