Google Events from old calendar sync persist - should be deleted

[Valentina R]

When changing the synced Calendar ID or disconnecting the Google Account and setting up a new one, the previously synced events should be removed.

To reproduce version 1:

1. set up the global sync, add events, sync.
2. Then, select another calendar ID and add events to it. Both events are synced (even from the original sync).
3. Switch to the first cal ID that was synced, still both events persist.

To reproduce version 2:

1. Set up the global sync, add events, sync.
2. Then, disconnect and set up a new connection with a different Google Account. Add events and sync. Events from the old one are still visible, just like the new ones.

I haven’t tested with the staff calendars but it might be the same.

Reported here.

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[Valentina R]

Duplicate of #620.