Get rid of obsolete methods

Summary:
Some current methods in the Cantor code are obsolete.

Cf. https://build.kde.org/job/Applications/job/cantor/job/kf5-qt5%20FreeBSDQt5.13/13/warnings12Result/NORMAL/category.2119183113/

Test Plan:
1.Compile Cantor (compilation should end without warnings with Qt 5.13).

2. Open Cantor. Try to enter some commands.

Reviewers: KDE Edu, sirgienko

Reviewed By: sirgienko

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D22560