Summary: khtml is going away, so remove the classic mode that relies on it

Test Plan: It's optional, so it can go without much being lost

Reviewers: vkrause, plasma-devel, #plasma, bcooksley, gladhorn

Reviewed By: #plasma, gladhorn

Subscribers: gladhorn, nicolasfella, broulik, davidedmundson, ognarb, ngraham, #plasma, GB_2, sitter

Tags: #plasma

Maniphest Tasks: T11542

Differential Revision: https://phabricator.kde.org/D23799