Remove then branch of always false condition

Summary: The condition in OctaveExpresssion::parseError never comes true, so remove then branch.

Reviewers: Cantor, asemke

Reviewed By: Cantor, asemke

Subscribers: KDE Edu, Cantor

Tags: Cantor, KDE Edu

Differential Revision: https://phabricator.kde.org/D11965