migrated OC_Preferences::deleteAppFromAllUsers to deleteApp in \OCP\ICon...

...fig

cc @LukasReschke @DeepDiver1975 @PVince81 @nickvergessen 

Or should we keep the old methods and call the methods from the new config object?
Yes. https://github.com/owncloud/core/blob/master/tests/lib/appconfig.php#L176:L182