backward compatibility not needed here

tools/lammps-gui/flagwarnings.h

@@ -32,13 +32,8 @@ protected:
     void highlightBlock(const QString &text) override;
 
 private:
-#if QT_VERSION < QT_VERSION_CHECK(5, 15, 0)
-    QRegExp isWarning;
-    QRegExp isURL;
-#else
     QRegularExpression isWarning;
     QRegularExpression isURL;
-#endif
     QTextCharFormat formatWarning;
     QTextCharFormat formatURL;
     QLabel *summary;