clean flag should also work if the source is not accessible / does not exist anymore

[lukasjarosch]

I've recently switched customers and my access to the source calendar was revoked. Unfortunately the--clean flag does not work without a source calendar / without access to a source calendar.

I would expect that if I remove the source key from my config / the source calendar is not accessible and run Calendarsync with the clean flag, that all events from said source are removed.