o don't delete the used style manager as it can be a shared style manager created and owned by the application
o don't create a style manager for the text shape if there is none.

This fixes crashes due to deleting the style manager of the application.

This also means that if an application wants to use a style manager it has to provide one. Otherwise no style manager is used. I think there is still some code that needs to be updated so that it can work that way also e.g KoTextShapeData::saveOdf.