Remove three classes form the SessionController header/source

Summary:
Those three classes where *used* inside of the SessionController
but they where not supposed to be there. The code in the
SessionController is already too big to understand, so the removal
of unrelated code is a good thing: it's now easier to reason
about those other three classes without having to deal with
the whole code that the SessionController has.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17485