Remove QT_STRINGVIEW_LEVEL macro

The idea of the macro was to be able to build Qt in different levels of QStringView "immersiveness" to benchmark the impact, in particular, of replacing QString with QStringView functions instead of just overloading them.

We never did that measurement when it could have had a meaningful effect (before 6.0) and the premiss of the macro turned out to be wrong, too: because we don't allow QStringView construction from anything that implicitly converts to QString, the QString partner overloads are still required.

QAnyStringView fixes that particular issue, but it doesn't allow overloading with QString, so we can't have a similar macro for QAnyStringView.

In essence, the QT_STRINGVIEW_LEVEL macro is unused and unusable, and we should eradicate it from the code base.