Move kactivitymanagerd to libexec and remove manual daemon management

Summary:
Since it was made DBus activatable having any other code to start/stop
the daemon doesn't really make any sense. For that reason I've
moved it to libexec.

Also the original code can be more efficient. It creates a whole qapp
instance, checks if we're registered then forks itself to spawn a new
version of the app that then registers otherwise quits.

KDBusService already does all this in a simpler faster non-racey way.

Test Plan:
Installed
Still got kactivitymanagerd

Reviewers: Plasma, mart

Reviewed By: Plasma, mart

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D15870