[Calculator Runner] Minor code optimisation

Summary:
Remove redunant QString::contains() checks.

Create one QRegularExpression object and use setPattern().

Test Plan: make && ctest

Reviewers: Plasma, apol, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28132