remove qt4

Summary: qt4 is long maintained for years and it's becoming a problem for us to not

break compatibility becuase we don't even remember what c++ standard we used for qt4

Test Plan: builds and (seems) to install relevant files. phonon-vlc still builds as well

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: asturmlechner

Differential Revision: https://phabricator.kde.org/D22688