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