Deprecate KBackup::backupFile() due to lost functionality
No-one bothered since KF 5.0 to care for the
#pragma message("KDE5 TODO: Remove KConfig correctly")
sadly. Krita just ported away already, Calligra core, Plan & Tellico still use the broken method without knowing of the case possibly (or just have not really used the config functionality). The warning should help them to make up their mind.