Use Q_ASSERT to catch null pointers for _sessionController and session()

Summary:
Remove TerminalDisplay::sessionIsPrimaryScreen(), and use Q_ASSERT
instead, where if _sessionController is actually a nullptr it means
the current konsole process is doomed anyway. Better catch such cases in
debug builds.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D13271