CPlusPlus: Remove Rreprocessor::run overload for QStrings

The preprocessor operates on QByteArray, making it less
convenient to use strings helps preventing accidental
conversion roundtrips.

Change-Id: Ifb2068a8fed137c52b05f2979b99cbce3462151e
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/libs/cplusplus/pp-engine.cpp

@@ -748,11 +748,6 @@ Preprocessor::Preprocessor(Client *client, Environment *env)
 {
 }
 
-QByteArray Preprocessor::run(const QString &fileName, const QString &source)
-{
-    return run(fileName, source.toUtf8());
-}
-
 QByteArray Preprocessor::run(const QString &fileName,
                              const QByteArray &source,
                              bool noLines,