Removing dead code - activity-event-handling private slots

Summary:
A few private slots handling the events of Activity objects
have been lingering in ShellCorona never to be called.
I've grepped both plasma-desktop and plasma-workspace for these
(even though they are private slots) and they seem not to be
used anywhere.

Reviewers: Plasma, sebas, davidedmundson, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2174