mainwindow: Correctly remove cards when they go away
The reference is owned by the VBox, so we remove it there and it gets cleaned up, like other widgets.
Fixes: #162 (closed)
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Equinix is shutting down its operations with us on April 30, 2025. They have graciously supported us for almost 5 years, but all good things come to an end. We are expecting to transition to new infrastructure between late March and mid-April. We do not yet have a firm timeline for this, but it will involve (probably multiple) periods of downtime as we move our services whilst also changing them to be faster and more responsive. Any updates will be posted in freedesktop/freedesktop#2011 as it becomes clear, and any downtime will be announced with further broadcast messages.
The reference is owned by the VBox, so we remove it there and it gets cleaned up, like other widgets.
Fixes: #162 (closed)