[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