Remove Qt5WebKitWidgets dependency and build gui unconditionally.
Summary: Qt5WebKitWidgets is not actually used anywhere.
Reviewers: sgaist, Plasma, davidedmundson, graesslin
Reviewed By: Plasma, davidedmundson, graesslin
Subscribers: graesslin, davidedmundson
Differential Revision: https://phabricator.kde.org/D1919