Drop support for pragma Singleton in Theme.qml
As stated in the comment, this automatic fix-up was scheduled for removal in KF6. Styles are responsible for providing correct Theme definitions on their own.
This cleanup and the process of testing it with various styles makes me think, it Theme.qml is so special, maybe is it should be named a bit more magical? For example, in lowercase, or with special characters? It would be less confusing if the filename conveyed the idea that it is never imported and instantiated as a module component.