Remove virtual for non-virtual calls

Summary:
TerminalDisplay is internal to Konsole, and it's never inherited,
that means that the 'virtual' on those functions are dummy and
does nothing besides increase the VTable size.

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17222