Fix warning about unused variable and minor optimization

Fixes this warning:
katescriptmanager.cpp: In member function ‘virtual bool KateScriptManager::exec(KTextEditor::View*, const QString&, QString&, const KTextEditor::Range&)’:
katescriptmanager.cpp:309:85: warning: unused parameter ‘errorMsg’ [-Wunused-parameter]

309 | bool KateScriptManager::exec(KTextEditor::View *view, const QString &_cmd, QString &errorMsg, const KTextEditor::Range &)
    |                                                                            ~~~~~~~~~^~~~~~~~

In addition, avoid QString construction by using the QStringRef
for == string comparison.