Remove automatic agent restarting when the agent executable changes

Summary:
This was intended as a way to help agent developers, but nowadays it most
of the time just triggers for kdesrc-build users while the entire system
is still in an inconsistent state when the agent can't be started again
yet.

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D15346