Make the policies obsolete

Unfortunately, this is a bit bad heritage of the QSerialDevice project. It is not well-thought, and it was added in rush.

If this feature ever comes back, it has to be rethought and maybe redesigned. Currently, it is not much usable nicely.

If we deprecate it now with QT_DEPRECATED similarly to this: https://codereview.qt-project.org/#patch,all,43441,7

users will get a warning, and they will hopefully stop using it. Then, we can redesign it for 5.3, or even completely remove it hopefully, but this has to be discussed with Lars, too, if he agrees with it overall since that would be an unusual Qt decision even if it is just an add-on.

Let us see how it goes, but the first step is deprecate it...