fix activity runner

Summary:
Activity runner is now useless:

m_activities is always nullptr in ActivityRunner::run because ActivityRunner::down is called after ActivityRunner::match which makes activity changing impossible

Reviewers: broulik, ngraham, davidedmundson

Reviewed By: ngraham, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27367